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Linear Regression Models (linear + regression_models)
Kinds of Linear Regression Models Selected AbstractsComparison of Linear Regression Models for Quantitative Geochemical Analysis: An Example Using X-Ray Fluorescence SpectrometryGEOSTANDARDS & GEOANALYTICAL RESEARCH, Issue 3 2005Mirna Guevara calibration analytique; régression linéaire; matériaux de référence en géochimie; géochimie analytique; loi de propagation d'erreurs This paper presents statistical aspects related to the calibration process and a comparison of different regression approaches of relevance to almost all analytical techniques. The models for ordinary least-squares (OLS), weighted least-squares (WLS), and maximum likelihood fitting (MLF) were evaluated and, as a case study, X-ray fluorescence (XRF) calibration curves for major elements in geochemical reference materials were used. The results showed that WLS and MLF models were statistically more consistent in comparison with the usually applied OLS approach. The use of uncertainty on independent and dependent variables during the calibration process and the calculation of final uncertainty on individual results using error propagation equations are the novel aspects of our work. Cet article présente les aspects statistiques liés au processus de calibration et fait une comparaison des différents calculs de régression utilisés dans pratiquement toutes les techniques analytiques. Les modèles des moindres carrés ordinaires (MCO) et pondérés (MCP), et d'ajustement de maximum de vraisemblance (AMV) ont étéévalués et appliqués aux courbes de calibration d'éléments majeurs obtenues en analyse par fluorescence X (XRF) de matériaux certifiés de référence. Les résultats obtenus avec les modèles MCP et AMV sont plus cohérents statistiquement que ceux obtenus la méthode classique des MCO. L'utilisation de l'incertitudes sur des variables indépendantes ou dépendantes durant la procédure de calibration et le calcul de l'incertitude finale sur chaque résultat à partir des lois de propagation d'erreur sont des aspects novateurs de ce travail. [source] Imputation and Variable Selection in Linear Regression Models with Missing CovariatesBIOMETRICS, Issue 2 2005Xiaowei Yang Summary Across multiply imputed data sets, variable selection methods such as stepwise regression and other criterion-based strategies that include or exclude particular variables typically result in models with different selected predictors, thus presenting a problem for combining the results from separate complete-data analyses. Here, drawing on a Bayesian framework, we propose two alternative strategies to address the problem of choosing among linear regression models when there are missing covariates. One approach, which we call "impute, then select" (ITS) involves initially performing multiple imputation and then applying Bayesian variable selection to the multiply imputed data sets. A second strategy is to conduct Bayesian variable selection and missing data imputation simultaneously within one Gibbs sampling process, which we call "simultaneously impute and select" (SIAS). The methods are implemented and evaluated using the Bayesian procedure known as stochastic search variable selection for multivariate normal data sets, but both strategies offer general frameworks within which different Bayesian variable selection algorithms could be used for other types of data sets. A study of mental health services utilization among children in foster care programs is used to illustrate the techniques. Simulation studies show that both ITS and SIAS outperform complete-case analysis with stepwise variable selection and that SIAS slightly outperforms ITS. [source] Evaluation of a bedside blood ketone sensor: the effects of acidosis, hyperglycaemia and acetoacetate on sensor performanceDIABETIC MEDICINE, Issue 7 2004A. S. A. Khan Abstract Aims To assess the performance of a handheld bedside ketone sensor in the face of likely metabolic disturbances in diabetic ketoacidosis, namely: pH, glucose and acetoacetate. Methods The effects of pH (7.44,6.83), glucose (5,50 mmol/l) and acetoacetate (0,5 mmol/l) were examined in venous blood to investigate the accuracy of betahydroxybutyrate measurement (0,5 mmol/l) by a handheld ketone sensor. Sensor results were compared with a reference method. Linear regression models were fitted to the difference between the methods with the concentration of metabolite as the explanatory factor. Results Decreasing pH and increasing glucose had no effect on the accuracy of the handheld ketone sensor; the gradients of the fitted lines were ,0.14 and ,0.003, respectively. The 95% confidence intervals were ,0.7,0.4 and ,0.01,0.004, respectively (P = 0.59 and 0.4, respectively). In the acetoacetate study, a positive relationship between the sensor and reference method results was found, the gradient was 0.09. The 95% confidence interval was 0.05,0.14 (P , 0.001), indicating that high concentrations of acetoacetate interfere with the sensor performance. Conclusions Acidosis and hyperglycaemia have minimal effects on the sensor performance. However, high concentrations of acetoacetate result in some overestimation of betahydroxybutyrate. This bedside ketone sensor provides useful data over a broad range of conditions likely to be encountered during moderate to severe diabetic ketoacidosis. [source] Landscape influences on aluminium and dissolved organic carbon in streams draining the Hubbard Brook valley, New Hampshire, USAHYDROLOGICAL PROCESSES, Issue 9 2005Sheila M. Palmer Abstract Concentrations of both aluminium (Al) and dissolved organic carbon (DOC) in stream waters are likely to be regulated by factors that influence water flowpaths and residence times, and by the nature of the soil horizons through which waters flow. In order to investigate landscape-scale spatial patterns in streamwater Al and DOC, we sampled seven streams draining the Hubbard Brook valley in central New Hampshire. We observed considerable variation in stream chemistry both within and between headwater watersheds. Across the valley, concentrations of total monomeric aluminium (Alm) ranged from below detection limits (<0·7 µmol l,1) to 22·3 µmol l,1. In general, concentrations of Alm decreased as pH increased downslope. There was a strong relationship between organic monomeric aluminium (Alo) and DOC concentrations (R2 = 0·92). We observed the highest Alm concentrations in: (i) a watershed characterized by a steep narrow drainage basin and shallow soils and (ii) a watershed characterized by exceptionally deep forest floor soils and high concentrations of DOC. Forest floor depth and drainage area together explained much of the variation in ln Alm (R2 = 0·79; N = 45) and ln DOC (R2 = 0·87; N = 45). Linear regression models were moderately successful in predicting ln Alm and ln DOC in streams that were not included in model building. However, when back-transformed, predicted DOC concentrations were as much as 72% adrift from observed DOC concentrations and Alm concentrations were up to 51% off. This geographic approach to modelling Al and DOC is useful for general prediction, but for more detailed predictions, process-level biogeochemical models are required. Copyright © 2005 John Wiley & Sons, Ltd. [source] Statistical probability distribution of snow depth at the model sub-grid cell spatial scaleHYDROLOGICAL PROCESSES, Issue 2 2005Dr Wolf-Dietrich Marchand Abstract Comprehensive snow depth data, collected using georadar and hand probing, were used for statistical analyses of snow depths inside 1 km grid cells. The sub-grid cell spatial scale was 100 m. Statistical distribution functions were found to have varying parameters, and an attempt was made to connect these statistical parameters to different terrain variables. The results showed that the two parameters mean and standard deviation of snow depth were significantly related to the sub-grid terrain characteristics. Linear regression models could explain up to 50% of the variation for both of the snowcover parameters mentioned. Copyright © 2004 John Wiley & Sons, Ltd. [source] Annual temperature history in Southwest Tibet during the last 400 years recorded by tree ringsINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2010Bao Yang Abstract We present a tree ring-width record from the southern Tibetan Plateau (TP) which spans from 1612,1998 AD (387 years). The series was developed from Tibetan juniper (Juniperus tibetica Kom) growing at sites near the western distribution limit of the species. Two versions of the chronology, a traditionally standardized chronology (TSC) and a regional curve standardization (RCS)chronology were developed. Linear regression models between ring width and mean annual temperature account for 41% (TSC) and 43% (RCS) of the annual (July,June) temperature variance for the period 1957,1998. According to the TSC reconstruction, warm periods occurred during the 1620s, 1650,1675, 1720s, 1740,1790, 1810s, 1850s,1890s, 1935,1950, and 1957,1964 and since 1980. Cold conditions prevailed during the 1630s,1640s, 1680s,1710s, 1730s, 1820,1840s, 1900s,1920s and the 1970s. Within the last 400 years, the late-20th century warming is distinctive but still within the range of natural climatic variability of this region. Comparison of our TSC reconstruction with proxy temperature records from other parts of the TP shows that the cold conditions during the 1730s, 1900s,1920s, and 1970s, and the warm periods during the 1770,1800, 1850s,1890s, 1935,1950, and 1957,1964 and since 1980 were synchronously occurring broad-scale climate anomalies on the whole TP. Differences between the reconstructions are found during the 17th century and around 1760, which were probably caused by local differences in temperature change and different sensitivity in seasonality. The RCS series portrays low-frequency variations such as warm periods during 1620,1640, 1650,1690, 1715,1790, and 1845,1875, and cold conditions during 1640,1650, 1690,1715, and 1875,1995. These long-term trends need to be verified by developing other proxy records that target to capture low-frequency signals in the future. Copyright © 2009 Royal Meteorological Society [source] Evaluating cognition in an elderly cohort via telephone assessmentINTERNATIONAL JOURNAL OF GERIATRIC PSYCHIATRY, Issue 5 2010Effie M. Mitsis Abstract Objective Longitudinal neuropsychological assessment provides the opportunity to observe the earliest transition to cognitive impairment in healthy, elderly individuals. We examined the feasibility, and its comparability to in-person assessment, of a telephone administered battery of established neuropsychological measures of cognitive functioning in healthy, elderly women. Methods Fifty-four women (age,=,79,±,7.7; education,=,15.4,±,3.3) who were in self-reported good health were recruited from senior centers and other community sources. A two-way cross-over design was used in which participants were randomly assigned to receive either (1) in-person neuropsychological assessment followed by telephone assessment and (2) telephone assessment followed by in-person assessment, separated by approximately 4 weeks. Linear regression models were used to determine whether there were performance differences by method (in-person vs. telephone), and equivalence testing assessed comparability of the two methods. Results There were no statistically significant differences in performance between in-person and telephone assessments on most neuropsychological tests, with the exception of digit span backward, Oral Trail Making Test Part A, and delayed recall on the SRT, the latter likely related to non-comparable exposure (6-trials in-person vs. 3-trials telephone). Equivalence testing differences fell in the pre-specified clinical equivalence zones, providing evidence of comparability of the two methods. Conclusions These pilot data support telephone administration of a neuropsychological battery that yields comparable performance to in-person assessment with respect to most instruments. Significant differences in scores on some measures suggest care should be taken in selecting specific measures used in a neuropsychological battery administered by telephone. Copyright © 2009 John Wiley & Sons, Ltd. [source] Complexity Prediction Instrument to detect ,complex cases' in respiratory wards: instrument developmentJOURNAL OF ADVANCED NURSING, Issue 1 2008Elena Lobo Abstract Title.,Complexity Prediction Instrument to detect ,complex cases' in respiratory wards: instrument development. Aim., This paper is a report of a study to test the hypothesis that the Spanish version of the Complexity Prediction Instrument is a reliable and valid measure of complexity of patients with respiratory disease and to identify the frequency of positive indicators of potential complexity. Background., Respiratory patients are often disabled and severely ill, with co-morbid physical conditions and associated psychosocial problems and need complex nursing care. Method., Trained nurses assessed 299 consecutive adult patients admitted to a respiratory service in Spain from May 2003 until June 2004 with the new, Spanish version of the instrument. Criterion-related validity was tested by studying its ability to predict complexity of care in terms of: severity of illness, scored using the Cumulative Illness Rating Scale; length of hospital stay; ,multiple consultations' during admission; and ,multiple specialists' after discharge. Findings., The hypothesis was supported: patients rating above the standard cut-off point on the Complexity Prediction Instrument scored statistically significantly higher on most of the measures of care complexity studied. Linear regression models showed that the tool was associated with ,length of hospital stay', and predicted both ,multiple consultations' and ,multiple specialists', after controlling for potential confounders. The proportion of ,probable complex cases' was 59·5%. Five positive indicators of potential complexity had a frequency higher than 50%. Conclusion., The Complexity Prediction Instrument is reliable and valid in a new clinical area, respiratory disease. It may be used by nurses for the early prediction of complexity of care. International comparisons may be facilitated with this new Spanish version. [source] Association Between Dietary Quality of Rural Older Adults and Self-Reported Food Avoidance and Food Modification Due to Oral Health ProblemsJOURNAL OF AMERICAN GERIATRICS SOCIETY, Issue 7 2010Margaret R. Savoca PhD OBJECTIVES: To quantify the association between food avoidance and modification due to oral health problems, to examine the association between food practices and dietary quality, and to determine foods associated with these self-management behaviors. DESIGN: Cross-sectional. SETTING: Rural North Carolina. PARTICIPANTS: Six hundred thirty-five community-dwelling adults aged 60 and older. MEASUREMENTS: Demographic and food frequency data and oral health assessments were obtained during home visits. Avoidance (0, 1,2 foods, 3,14 foods) and modification (0,3 foods, 4,5 foods) due to oral health problems were assessed for foods representing oral health challenges. Food frequency data were converted into Healthy Eating Index-2005 (HEI-2005) scores. Linear regression models tested the significance of associations between HEI-2005 measures and food avoidance and modification. RESULTS: Thirty-five percent of participants avoided three to 14 foods, and 28% modified four to five foods. After adjusting for age, sex, ethnicity, poverty, education, and tooth loss, total HEI-2005 score was lower (P<.001) for persons avoiding more foods and higher for persons modifying more foods (P<.001). Those avoiding three to 14 foods consumed more saturated fat and energy from solid fat and added sugar and less nonhydrogenated fat than those avoiding fewer than three foods. Those who modified four to five foods consumed less saturated fat and solid fat and added sugar but more total grains than those modifying fewer than four foods. CONCLUSION: Food avoidance and modification due to oral health problems are associated with significant differences in dietary quality. Approaches to minimize food avoidance and promote food modification by persons having eating difficulties due to oral health conditions are needed. [source] Simple means to improve the interpretability of regression coefficientsMETHODS IN ECOLOGY AND EVOLUTION, Issue 2 2010Holger Schielzeth Summary 1. Linear regression models are an important statistical tool in evolutionary and ecological studies. Unfortunately, these models often yield some uninterpretable estimates and hypothesis tests, especially when models contain interactions or polynomial terms. Furthermore, the standard errors for treatment groups, although often of interest for including in a publication, are not directly available in a standard linear model. 2. Centring and standardization of input variables are simple means to improve the interpretability of regression coefficients. Further, refitting the model with a slightly modified model structure allows extracting the appropriate standard errors for treatment groups directly from the model. 3. Centring will make main effects biologically interpretable even when involved in interactions and thus avoids the potential misinterpretation of main effects. This also applies to the estimation of linear effects in the presence of polynomials. Categorical input variables can also be centred and this sometimes assists interpretation. 4. Standardization (z -transformation) of input variables results in the estimation of standardized slopes or standardized partial regression coefficients. Standardized slopes are comparable in magnitude within models as well as between studies. They have some advantages over partial correlation coefficients and are often the more interesting standardized effect size. 5. The thoughtful removal of intercepts or main effects allows extracting treatment means or treatment slopes and their appropriate standard errors directly from a linear model. This provides a simple alternative to the more complicated calculation of standard errors from contrasts and main effects. 6. The simple methods presented here put the focus on parameter estimation (point estimates as well as confidence intervals) rather than on significance thresholds. They allow fitting complex, but meaningful models that can be concisely presented and interpreted. The presented methods can also be applied to generalised linear models (GLM) and linear mixed models. [source] Performance measures in Friedreich ataxia: Potential utility as clinical outcome toolsMOVEMENT DISORDERS, Issue 7 2005David R. Lynch MD Abstract Although several neuroprotective agents have been proposed as potential therapies in Friedreich ataxia (FA), clinical trials of their efficacy are limited by a lack of sensitive outcome measures. We assessed whether performance measures (nine-hole peg test, the timed 25-foot walk, and low-contrast letter acuity) provide valid measures of disease status in FA. Scores for each measure correlated significantly with neurologic disability and disease duration. Rank correlations between scores for performance measures were moderate in magnitude, suggesting that the each test captures separate yet related dimensions of neurological function in FA. Linear regression models demonstrated that scores from the nine-hole peg test and the timed 25-foot walk (after reciprocal transformation) were predicted by age and triplet repeat length in patients with FA. In addition, comparison of the temporal courses of change for each performance measure demonstrated that scores from the timed 25-foot walk change early in the course of FA, nine-hole peg test scores change slowly over the full course of the disorder, and low-contrast letter acuity scores change in the later stages of the disease. Thus, a composite scale derived from these performance measures may provide the best overall measure for assessing disease progression throughout the illness. © 2005 Movement Disorder Society [source] Relationship Between Post-traumatic Stress Disorder and Pain in Two American Indian TribesPAIN MEDICINE, Issue 1 2005Dedra Buchwald MD ABSTRACT Objectives., To estimate the association of lifetime post-traumatic stress disorder (PTSD) and pain in American Indians, and determine if tribe, sex, cultural and psychosocial factors, or major depression influence the magnitude of this association. Design., A cross-sectional probability sample survey completed between 1997 and 2000. A structured interview was conducted by trained, tribal members to gather information on demographic and cultural features, physical health status, psychiatric disorders, and functional status. Setting., General community. Participants., A total of 3,084 individuals randomly selected from the tribal rolls of a Southwestern (N = 1,446) and a Northern Plains (N = 1,638) tribal group who were 15,54 years of age and lived on or within 20 miles of their reservations. Outcome Measures., Bodily pain subscale of the Short Form-36. Linear regression models were fit to examine the association between lifetime PTSD and pain, adjusting for demographic, cultural, psychosocial features, painful medical conditions, and major depression. Results., The prevalence of lifetime PTSD was 16% in the Southwestern and 14% in the Northern Plains; women were nearly twice as likely as men to have lifetime PTSD in both tribes. The final adjusted model demonstrated that mean Short Form-36 bodily pain subscale scores were lower (indicating more pain) among individuals with lifetime PTSD than those without lifetime PTSD. Effect modification by tribe, sex, and depression was not observed. Conclusions., Lifetime PTSD was strongly associated with bodily pain in this rural sample of American Indians. Clinicians should be aware of, and address, the link between physical pain syndromes and PTSD. [source] The Use of an Exact Light-Scattering Theory for Spheroidal TiO2 Pigment ParticlesPARTICLE & PARTICLE SYSTEMS CHARACTERIZATION, Issue 2 2006Juho-Pertti Jalava Abstract Titanium dioxide pigments are widely used, e.g., in paints, printing inks, plastics, and papers, because of the excellent and whiteness they provide. As is well known, the opacity and to it related optical properties depend on the pigment's crystal- and particle-size distributions (CSD, PSD). Therefore, the process and quality control of the CSD and PSD are very essential. The development of a straightforward and sophisticated method for process and quality control is presented here. The principal component is a new determination method for the PSD based on an accurate theory of light scattering of spheroidal particles. In this new method, called the turbidity spectrum method (TSM), the whole turbidity spectrum is measured. This spectrum, which is due to the particle-size distribution of the pigment particles, is then solved using the T-matrix method by taking into account the shape of the particles. After that the CSD is extracted from the PSD. The T-matrix method is one of the most powerful and widely used tools for the rigorous computing of electromagnetic scattering by non-spherical particles. The repeatability of the mean PSD (,200,nm) obtained by the TSM is excellent, with a standard deviation of 0.1,nm (0.05,%). Linear regression models based on TSM results were developed for undertone and tint-reducing power, which are important optical parameters of the pigment. The excellent repeatability of the TSM results was necessary for the production of representative models. [source] Approaches for assessing the role of household socioeconomic status on child anthropometric measures in urban South AfricaAMERICAN JOURNAL OF HUMAN BIOLOGY, Issue 1 2009Zoë A. Sheppard The objectives of this article were to compare the variance explained in anthropometric outcomes when using individual measures of socioeconomic status (SES) versus different approaches to create SES indices within the urban African context, and to examine the influence of SES measured during infancy on child anthropometric outcomes at 7/8 years. Data from the 1990 Birth-to-Twenty cohort study set in Johannesburg-Soweto, South Africa, were used (n = 888). Linear regression models were used to investigate the association between SES (individual and index measures) during infancy and anthropometric measures at age 7/8 years, controlling for sex, age, and population group. Both individual and index measures of SES explained similar proportions of the variance for each anthropometric outcome. SES measured during infancy influenced weight more than height at age 7/8 years in Johannesburg-Soweto. Positive associations were found between SES and the anthropometric measures,,ownership of a car, telephone, and having an inside flush toilet were the most significant SES variables. The similarities observed in the variance explained relating to the anthropometric outcomes suggest that researchers who want to adjust for SES in analyses could use an SES index to make statistical models more parsimonious. However, using such indices loses information relating to the specific socioeconomic factors that are important for explaining child anthropometrics. If the purpose of the research is to make policy recommendations for the improvement of child growth, individual SES variables would provide more specific information to target interventions. Am. J. Hum. Biol., 2009. © 2008 Wiley-Liss, Inc. [source] Effectiveness of Increasing Emergency Department Patients' Self-perceived Risk for Being Human Immunodeficiency Virus (HIV) Infected Through Audio Computer Self-interview,based Feedback About Reported HIV Risk BehaviorsACADEMIC EMERGENCY MEDICINE, Issue 11 2009Roland C. Merchant MD Abstract Objectives:, Prior research has demonstrated that emergency department (ED) patient acceptance of human immunodeficiency virus (HIV) screening is partially dependent on patients' self-perceived risk of infection. The primary objective of this study was to determine the effectiveness of audio computer-assisted self-interview (ACASI)-based feedback. The intervention aimed to increase patient's self-perceived risk of being HIV infected by providing immediate feedback on their risk behaviors. Methods:, This 1-year, randomized, controlled trial at a U.S. ED enrolled a random sample of 18- to 64-year-old subcritically ill or injured adult patients who were not known to be HIV infected. All participants completed an anonymous, ACASI-based questionnaire about their HIV risk behaviors related to injection drug use and sex, as well as their self-perceived risk for being HIV infected. Participants were randomly assigned to one of two study groups: an intervention group in which participants received immediate ACASI-based feedback in response to each of their reported risk behaviors or a no-intervention group without feedback. Participants were asked to indicate their level of HIV risk on a five-point scale before and after they answered the questions. Change in level of self-perceived HIV risk was calculated and compared by study group using Pearson's chi-square test. An HIV risk behavior score that summarized reported HIV risk behavior was devised. Because HIV risk behaviors differ by sex, scores were calculated separately for each sex. Linear regression models that adjusted for study group and same subject covariance were employed to determine if higher HIV risk behavior scores were associated with an increase in self-perceived HIV risk. Results:, Of the 566 trial participants, the median age was 29 years (interquartile range [IQR] = 22,43 years), 62.2% were females, and 66.9% had been tested previously for HIV. After answering the reported HIV risk behavior questions, 12.6% of participants had an increase, 79.9% had no change, and 7.5% had a decrease in self-perceived HIV risk. Of the 46.6% of participants who initially indicated that they were not at risk for HIV, 11.4% had an increase in self-perceived HIV risk after answering the reported HIV risk behavior questions. Change in self-perceived HIV risk did not differ by study group (p = 0.77). There were no differences in reported HIV risk scores between the intervention and no-intervention groups for females (p = 0.78) or males (p = 0.86). In the linear regression models, a greater increase in self-perceived HIV risk was associated with higher reported HIV risk behavior scores for females (, = 0.59, 95% confidence interval [CI] = 0.15, 1.04) but not for males (, = 1.00, 95% CI = ,0.13 to 2.14). Conclusions:, Some ED patients can be moved, although modestly, to recognize their risk for being HIV infected by asking about their HIV risk behaviors. However, ACASI-based feedback messages about HIV risk behaviors do not increase subjects' self-perceived HIV risk. Female ED patients appear to increase their self-perceived HIV risk more than males when queried about their HIV risk behaviors. [source] Video as an Effective Method to Deliver Pretest Information for Rapid Human Immunodeficiency TestingACADEMIC EMERGENCY MEDICINE, Issue 2 2009Roland C. Merchant MD Abstract Objectives:, Video-based delivery of human immunodeficiency virus (HIV) pretest information might assist in streamlining HIV screening and testing efforts in the emergency department (ED). The objectives of this study were to determine if the video "Do you know about rapid HIV testing?" is an acceptable alternative to an in-person information session on rapid HIV pretest information, in regard to comprehension of rapid HIV pretest fundamentals, and to identify patients who might have difficulties in comprehending pretest information. Methods:, This was a noninferiority trial of 574 participants in an ED opt-in rapid HIV screening program who were randomly assigned to receive identical pretest information from either an animated and live-action 9.5-minute video or an in-person information session. Pretest information comprehension was assessed using a questionnaire. The video would be accepted as not inferior to the in-person information session if the 95% confidence interval (CI) of the difference (,) in mean scores on the questionnaire between the two information groups was less than a 10% decrease in the in-person information session arm's mean score. Linear regression models were constructed to identify patients with lower mean scores based upon study arm assignment, demographic characteristics, and history of prior HIV testing. Results:, The questionnaire mean scores were 20.1 (95% CI = 19.7 to 20.5) for the video arm and 20.8 (95% CI = 20.4 to 21.2) for the in-person information session arm. The difference in mean scores compared to the mean score for the in-person information session met the noninferiority criterion for this investigation (, = 0.68; 95% CI = 0.18 to 1.26). In a multivariable linear regression model, Blacks/African Americans, Hispanics, and those with Medicare and Medicaid insurance exhibited slightly lower mean scores, regardless of the pretest information delivery format. There was a strong relationship between fewer years of formal education and lower mean scores on the questionnaire. Age, gender, type of insurance, partner/marital status, and history of prior HIV testing were not predictive of scores on the questionnaire. Conclusions:, In terms of patient comprehension of rapid HIV pretest information fundamentals, the video was an acceptable substitute to pretest information delivered by an HIV test counselor. Both the video and the in-person information session were less effective in providing pretest information for patients with fewer years of formal education. [source] Significant inverse relationship between serum free T4 concentration and body mass index in euthyroid subjects: differences between smokers and nonsmokersCLINICAL ENDOCRINOLOGY, Issue 4 2008Ashley E. Makepeace Summary Objective, There are conflicting data regarding the relationship between thyroid function and body mass index (BMI) in euthyroid subjects, and it is uncertain whether tobacco smoking modifies this relationship. The objective of this study was to examine the relationships between thyroid function, BMI and smoking in euthyroid subjects. Design, Linear regression models were used to examine the relationships between serum free T4, serum TSH, BMI and smoking in a cross-sectional, community-based sample of 1853 euthyroid subjects in Busselton, Western Australia. Results, There was a significant negative relationship between free T4 and BMI: after adjustment for age and sex, each 1 pmol/l increase in free T4 was associated with a decrease in BMI of 0·12 kg/m2 (95% CI 0·06, 0·18; P < 0·001). The mean BMI ± SD of subjects in the highest quintile of free T4 concentration was 24·4 ± 3·5 kg/m2, compared with 26·1 ± 3·8 kg/m2 for the lowest quintile. The relationship between free T4 and BMI was statistically significant (adjusted for age and sex) in subjects who had never smoked (P = 0·001) and former smokers (P = 0·011), but not in current smokers (P = 0·77). There was no significant relationship between TSH and BMI: after adjustment for age and sex, each 1 mU/l increase in TSH was associated with an increase in BMI of 0·08 kg/m2 (95% CI ,0·16, 0·32; P = 0·53). Conclusions, In euthyroid subjects, small differences in free T4 are associated with differences in BMI. This relationship is not present in current smokers. We speculate that this may be relevant to weight changes associated with smoking cessation. [source] US state alcohol sales compared to survey data, 1993,2006ADDICTION, Issue 9 2010David E. Nelson ABSTRACT Aims Assess long-term trends of the correlation between alcohol sales data and survey data. Design Analyses of state alcohol consumption data from the US Alcohol Epidemiologic Data System based on sales, tax receipts or alcohol shipments. Cross-sectional, state annual estimates of alcohol-related measures for adults from the US Behavioral Risk Factor Surveillance System using telephone surveys. Setting United States. Participants State alcohol tax authorities, alcohol vendors, alcohol industry (sales data) and randomly selected adults aged , 18 years 1993,2006 (survey data). Measurements State-level per capita annual alcohol consumption estimates from sales data. Self-reported alcohol consumption, current drinking, heavy drinking, binge drinking and alcohol-impaired driving from surveys. Correlation coefficients were calculated using linear regression models. Findings State survey estimates of consumption accounted for a median of 22% to 32% of state sales data across years. Nevertheless, state consumption estimates from both sources were strongly correlated with annual r-values ranging from 0.55,0.71. State sales data had moderate-to-strong correlations with survey estimates of current drinking, heavy drinking and binge drinking (range of r-values across years: 0.57,0.65; 0.33,0.70 and 0.45,0.61, respectively), but a weaker correlation with alcohol-impaired driving (range of r-values: 0.24,0.56). There were no trends in the magnitude of correlation coefficients. Conclusions Although state surveys substantially underestimated alcohol consumption, the consistency of the strength of the association between sales consumption and survey data for most alcohol measures suggest both data sources continue to provide valuable information. These findings support and extend the distribution of consumption model and single distribution theory, suggesting that both sales and survey data are useful for monitoring population changes in alcohol use. [source] Energy input and zooplankton species richnessECOGRAPHY, Issue 6 2007Dag O. Hessen What are the relative contribution of temperature and solar irradiance as types of energy deliveries for species richness at the ecosystem level? In order to reveal this question in lake ecosystems, we assessed zooplankton species richness in 1891 Norwegian lakes covering a wide range in latitude, altitude, and lake area. Geographical variables could largely be replaced by temperature-related variables, e.g. annual monthly maximum temperature or growth season. Multivariate analysis (PCA) revealed that not only maximum monthly temperature, but also energy input in terms of solar radiation were closely associated with species richness. This was confirmed by stepwise, linear regression analysis in which lake area was also found to be significant. We tested the predictive power of the "metabolic scaling laws" for species richness by regressing Ln of species richness over the inverse of the air temperature (in Kelvin), corrected for the activation energy (eV) as predicted by the Boltzmann constant. A significant, negative slope of 0.78 for ln richness over temperature, given as 1/kT, was found, thus slightly higher than the range of slopes predicted from the scaling law (0.60,0.70). Temperature basically constrained the upper bound of species number, but it was only a modest predictor of actual richness. Both PCA-analysis and linear regression models left a large unexplained variance probably due to lake-specific properties such as catchment influence, lake productivity, food-web structure, immigration constraints or more stochastic effects. [source] Recruitment of burbot (Lota lota L.) in Lake Erie: an empirical modelling approachECOLOGY OF FRESHWATER FISH, Issue 3 2010M. A. Stapanian Stapanian MA, Witzel LD, Cook A. Recruitment of burbot (Lota lota L.) in Lake Erie: an empirical modelling approach. Ecology of Freshwater Fish 2010: 19: 326,337. Published 2010. This article is a US Government work and is in the public domain in the USA Abstract,,, World-wide, many burbot Lota lota (L.) populations have been extirpated or are otherwise in need of conservation measures. By contrast, burbot made a dramatic recovery in Lake Erie during 1993,2001 but declined during 2002,2007, due in part to a sharp decrease in recruitment. We used Akaike's Information Criterion to evaluate 129 linear regression models that included all combinations of one to seven ecological indices as predictors of burbot recruitment. Two models were substantially supported by the data: (i) the number of days in which water temperatures were within optimal ranges for burbot spawning and development combined with biomass of yearling and older (YAO) yellow perch Perca flavescens (Mitchill); and (ii) biomass of YAO yellow perch. Warmer winter water temperatures and increases in yellow perch biomass were associated with decreases in burbot recruitment. Continued warm winter water temperatures could result in declines in burbot recruitment, particularly in the southern part of the species' range. [source] Microemulsion electrokinetic chromatography of drugs varying in charge and hydrophobicity: I. Impact of parameters on separation performance evaluated by multiple linear regression modelsELECTROPHORESIS, Issue 1 2004Valérie Harang Abstract The separation of anionic, cationic and neutral drugs in microemulsion electrokinetic chromatography (MEEKC) was studied with a statistical experimental design. The concentration of sodium dodecyl sulfate (SDS, surfactant), 1-butanol (co-surfactant) and borate buffer and the factors Brij 35 (surfactant), 2-propanol (organic solvent) and cassette temperature were varied simultaneously, while the parameters pH (9.2), the concentration of octane (oil, 0.8% w/w), the voltage (10 kV) and the dimension of the fused-silica capillary, were kept constant. Eight different model substances were chosen with different hydrophobicities. Two of the analytes were positively charged, two were negatively charged, and the remaining four were neutral or close to neutral at the pH explored. The importance of each parameter on the separation window, the plate height and the retention factor for each of the analytes was studied by means of multiple linear regression (MLR) models. A new response was evaluated for anions, the quotient between the effective mobility in the microemulsion and the effective mobility in the corresponding buffer. Factors affecting selectivity changes were also explored, and it was found that SDS and 2-propanol had the largest effect on selectivity. [source] Anthropogenic disturbance affects the structure of bacterial communitiesENVIRONMENTAL MICROBIOLOGY, Issue 3 2010Duane Ager Summary Patterns of taxa abundance distributions are the result of the combined effects of historical and biological processes and as such are central to ecology. It is accepted that a taxa abundance distribution for a given community of animals or plants following a perturbation will typically change in structure from one of high evenness to increasing dominance. Subsequently, such changes in evenness have been used as indicators of biological integrity and environmental assessment. Here, using replicated experimental treehole microcosms perturbed with different concentrations of the pollutant pentachlorophenol, we investigated whether changes in bacterial community structure would reflect the effects of anthropogenic stress in a similar manner to larger organisms. Community structure was visualized using rank,abundance plots fitted with linear regression models. The slopes of the regression models were used as a descriptive statistic of changes in evenness over time. Our findings showed that bacterial community structure reflected the impact and the recovery from an anthropogenic disturbance. In addition, the intensity of impact and the rate of recovery to pre-perturbation structure were dose-dependent. These properties of bacterial community structures may potentially provide a metric for environmental assessment and regulation. [source] Predicting intra-urban variation in air pollution concentrations with complex spatio-temporal dependencies,ENVIRONMETRICS, Issue 6 2010Adam A. Szpiro Abstract We describe a methodology for assigning individual estimates of long-term average air pollution concentrations that accounts for a complex spatio-temporal correlation structure and can accommodate spatio-temporally misaligned observations. This methodology has been developed as part of the Multi-Ethnic Study of Atherosclerosis and Air Pollution (MESA Air), a prospective cohort study funded by the US EPA to investigate the relationship between chronic exposure to air pollution and cardiovascular disease. Our hierarchical model decomposes the space--time field into a "mean" that includes dependence on covariates and spatially varying seasonal and long-term trends and a "residual" that accounts for spatially correlated deviations from the mean model. The model accommodates complex spatio-temporal patterns by characterizing the temporal trend at each location as a linear combination of empirically derived temporal basis functions, and embedding the spatial fields of coefficients for the basis functions in separate linear regression models with spatially correlated residuals (universal kriging). This approach allows us to implement a scalable single-stage estimation procedure that easily accommodates a significant number of missing observations at some monitoring locations. We apply the model to predict long-term average concentrations of oxides of nitrogen (NOx) from 2005 to 2007 in the Los Angeles area, based on data from 18 EPA Air Quality System regulatory monitors. The cross-validated IR2 is 0.67. The MESA Air study is also collecting additional concentration data as part of a supplementary monitoring campaign. We describe the sampling plan and demonstrate in a simulation study that the additional data will contribute to improved predictions of long-term average concentrations. Copyright © 2009 John Wiley & Sons, Ltd. [source] Oceanic influence on the precipitation of the south-east of VenezuelaENVIRONMETRICS, Issue 3 2002Lelys Guenni Abstract The Caroní catchment located in the south-east of Venezuela accounts for 70 per cent of the total hydropower energy of the country. On a year to year basis, it has been shown that low frequency large scale ocean-atmosphere phenomena are highly coupled to the hydroclimatology of the region, El Niño-Southern Oscillation (ENSO) being a major forcing mechanism of climatic and hydrological anomalies. Regional differences in amplitude and timing are due to complex orographic interactions, land surface-atmosphere feedback mechanisms and the evolution of dominant synoptic meteorological conditions. A detailed analysis of the relationship between rainfall and several large scale ocean-atmospheric variables was carried out to determine the potential use of large scale climatic information as predictors of the rainfall anomalies over the region. The problem was tackled in two ways: (a) first a seasonal dynamic rainfall model was fitted to monthly rainfall for different locations. In this case rainfall is assumed as a normal variate w which has been transformed to account for its departure from normality and truncated to account for the positive probability mass of zero values, which corresponds to negative values of the normal variable. The time series of the model parameters and the macroclimatic variables are inspected for their potential relationship with local rainfall via the stochastic model. (b) Second, dynamic linear regression models between the macroclimatic variables as predictors and the rainfall anomalies as predictant were fitted to evaluate and quantify the significance of these dependencies. Consistent patterns are observed with the Tropical Atlantic and Pacific ocean temperature anomalies, in which a significant negative relationship has been present since 1976, indicating an overall decrease (increase) in rainfall when the Pacific and the Tropical Atlantic are warmer (colder) than normal. In all cases the results suggest that the relationships between rainfall anomalies and the macroclimatic variables are not constant with time. Copyright © 2002 John Wiley & Sons, Ltd. [source] A new index of habitat alteration and a comparison of approaches to predict stream habitat conditionsFRESHWATER BIOLOGY, Issue 10 2007BRIAN FRAPPIER Summary 1. Stream habitat quality assessment complements biological assessment by providing a mechanism for ruling out habitat degradation as a potential stressor and provides reference targets for the physical aspects of stream restoration projects. This study analysed five approaches for predicting habitat conditions based on discriminant function, linear regressions, ordination and nearest neighbour analyses. 2. Quantitative physical and chemical habitat and riparian conditions in minimally-impacted streams in New Hampshire were estimated using United States Environmental Protection Agency's Environmental Monitoring and Assessment Program protocols. Catchment-scale descriptors were used to predict segment-scale stream channel and riparian habitat, and the accuracy and precision of the different modelling approaches were compared. 3. A new assessment index comparing and summarizing the degree of correspondence between predicted and observed habitat based on Euclidean distance between the standardized habitat factors is described. Higher index scores (i.e. greater Euclidean distance) would suggest a greater deviation in habitat between observed conditions and expected reference conditions. As in most biotic indices, the range in index scores in reference sites would constitute a situation equivalent to reference conditions. This new index avoids the erroneous prediction of multiple, mutually exclusive habitat conditions that have confounded previous habitat assessment approaches. 4. Separate linear regression models for each habitat descriptor yielded the most accurate and precise prediction of reference conditions, with a coefficient of variation (CV) between predictions and observations for all reference sites of 0.269. However, for a unified implementation in regions where a classification-based approach has already been taken for biological assessment, a discriminant analysis approach, that predicted membership in biotic communities and compared the mean habitat features in the biotic communities with the observed habitat features, was similar in prediction accuracy and precision (CV = 0.293). 5. The best model had an error of 27% of the mean index value for the reference sites, indicating substantial room for improvement. Additional catchment characteristics not readily available for this analysis, such as average rainfall or winter snow-pack, surficial geological characteristics or past land-use history, may improve the precision of the predicted habitat features in the reference streams. Land-use history in New Hampshire and regional environmental impacts have greatly impacted stream habitat conditions even in streams considered minimally-impacted today; thus as regional environmental impacts change and riparian forests mature, reference habitat conditions should be re-evaluated. [source] Is it possible to identify early predictors of the future cost of chronic arthritis?FUNDAMENTAL & CLINICAL PHARMACOLOGY, Issue 1 2009The VErA project Abstract This study was conducted to identify early predictors of the total cost of inflammatory arthritis (IA). One hundred and eighty patients affected by undifferentiated arthritis (UA) or rheumatoid arthritis (RA) were included in the French Very Early rheumatoid Arthritis (VErA) cohort between 1998 and 2001. Health economic data for 2003 were collected using a patient self-questionnaire. Results were analysed in terms of direct, indirect and total costs in 2003 euros (2003,) for the population as a whole and in diagnostic subgroups. A payor perspective (the French National Health Insurance, in this case) was adopted. Multiple linear regression models were used to identify predictors of total cost from among the criteria assessed on recruitment. Results of the study showed that for the study population as a whole, the mean total cost was ,4700 per patient. The costs attributable to the RA and UA sub-groups were ,5928 and ,2424 per patient, respectively. In a univariate analysis, certain parameters were significantly correlated with a higher cost of illness. In the multivariate analysis, some of these parameters were further identified as being predictive of higher cost. Two strong significant, early predictors of total cost were identified: higher pain (P = 0.002) and the presence of rheumatoid factor (P = 0.004). In the RA sub-group, lower grip strength of the dominant hand (P = 0.039) was another predictor of the illness's subsequent economic impact. In conclusion, our data show that simple clinical and laboratory parameters can be used early in the course of IA to predict the condition's impact on healthcare budgets. [source] Risk adjusted resource utilization for AMI patients treated in Japanese hospitalsHEALTH ECONOMICS, Issue 4 2007Edward Evans Abstract Though risk adjustment is necessary in order to make equitable comparisons of resource utilization in the treatment of acute myocardial infarction patients, there is little in the literature that can be practically applied without access to clinical records or specialized registries. The aim of this study is to show that effective models of resource utilization can be developed based on administrative data, and to demonstrate a practical application of the same models by comparing the risk-adjusted performance of the hospitals in our dataset. The study sample included 1748 AMI cases discharged from 10 large, private teaching hospitals in Japan, between 10 April 2001 and 30 June 2004. Explanatory variables included procedures (CABG and PCI), length of stay, outcome, patient demographics, diagnosis and comorbidity status. Multiple linear regression models constructed for the study were able to account for 66.5, 27.7, and 58.4% of observed variation in total charges, length of stay and charges per day, respectively. The performance of models constructed for this study was comparable to or better than performance reported by other studies that made use of explanatory variables extracted from clinical data. The use of administrative data in risk adjustment makes broad scale application of risk adjustment feasible. Copyright © 2006 John Wiley & Sons, Ltd. [source] The Relationship between Health Plan Performance Measures and Physician Network Overlap: Implications for Measuring Plan QualityHEALTH SERVICES RESEARCH, Issue 4 2010Daniel D. Maeng Objective. To examine the extent to which health plan quality measures capture physician practice patterns rather than plan characteristics. Data Source. We gathered and merged secondary data from the following four sources: a private firm that collected information on individual physicians and their health plan affiliations, The National Committee for Quality Assurance, InterStudy, and the Dartmouth Atlas. Study Design. We constructed two measures of physician network overlap for all health plans in our sample and linked them to selected measures of plan performance. Two linear regression models were estimated to assess the relationship between the measures of physician network overlap and the plan performance measures. Principal Findings. The results indicate that in the presence of a higher degree of provider network overlap, plan performance measures tend to converge to a lower level of quality. Conclusions. Standard health plan performance measures reflect physician practice patterns rather than plans' effort to improve quality. This implies that more provider-oriented measurement, such as would be possible with accountable care organizations or medical homes, may facilitate patient decision making and provide further incentives to improve performance. [source] 5-Aminosalicylate therapy is associated with higher 6-thioguanine levels in adults and children with inflammatory bowel disease in remission on 6-mercaptopurine or azathioprineINFLAMMATORY BOWEL DISEASES, Issue 4 2006Scott Hande MD Abstract Background: Small uncontrolled trials have suggested that 5-aminosalicylate (5-ASA) medications increase 6-thioguanine nucleotide (6-TGn) levels in adults with Crohn's disease (CD) on azathioprine (AZA) or 6-mercaptopurine (6-MP), presumably through the inhibition of thiopurine methyltransferase (TPMT). We tested the theory that coadministration of 5-ASA agents with AZA/6-MP results in higher 6-TGn levels in a large cohort of children and adults with CD or ulcerative colitis (UC). Methods: A retrospective cohort study identified all children and adults treated for IBD with AZA/6-MP at 2 tertiary medical centers. Patients were included if their TPMT genotype was known and 6-TGn and 6-methymercaptopurine (6-MMP) levels had been obtained after 3 months of clinical remission at a stable dose of AZA/6-MP. 6-TGn and 6-MMP levels were compared between patients taking and those not taking 5-ASA medications through the use of linear regression models to identify and adjust for potentially confounding variables. Results: Of the 126 patients included, 88 were taking 5-ASA medications. Patients on 5-ASA agents had higher mean 6-TGn levels after adjustment for confounding variables (,6-TGn, 47.6 ± 21.8 pmol/8 × 108 red blood cells; P = 0.03). CD and TPMT heterozygosity was independently associated with higher 6-TGn levels (P = 0.01 and P = 0.03, respectively). 5-ASA exposure was not associated with a change in 6-MMP levels. Conclusions: We found that 5-ASA therapy is associated with higher 6-TGn levels in children and adults with IBD on 6-MP/AZA. TPMT inhibition may not explain this effect because 5-ASA exposure did not affect 6-MMP levels. The observed association of CD with higher 6-TGn levels is novel and needs to be verified in prospective studies. [source] The effect of solvents on the rate of catalytic hydrogenation of 6-ethyl-1,2,3,4-tetrahydroanthracene-9,10-dioneINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2008ch Fajt The rate of hydrogenation of 6-ethyl-1,2,3,4-tetrahydroanthracene-9,10-dione was investigated at 313 K and 0.1 MPa in 20 solvents. A multiple linear regression was used to describe the solvent effect. The regression of the reaction rates was carried out using two five-parameter linear regression models: the Abraham,Kamlet,Taft (AKT) and the Koppel,Palm (KP) model. After the elimination of the insignificant terms from the regression models, it was found that the basic character of the solvent and its Hildebrand cohesion energy density were the most important attributes influencing the hydrogenation rate. The analysis of both models led to the same conclusion. The resultant simplified AKT model gave closer fitting in comparison to the KP model. The results could facilitate the solvent selection for the industrial process of hydrogen peroxide production by the anthraquinone method with respect to the kinetics of anthraquinone hydrogenation. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 240,252, 2008 [source] | ||||||||||||||||||||||||||||||||||||||||