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Systematic Biases (systematic + biase)
Selected AbstractsThe effect of differential rater function over time (DRIFT) on objective structured clinical examination ratingsMEDICAL EDUCATION, Issue 10 2009Kevin McLaughlin Context, Despite the impartiality implied in its title, the objective structured clinical examination (OSCE) is vulnerable to systematic biases, particularly those affecting raters' performance. In this study our aim was to examine OSCE ratings for evidence of differential rater function over time (DRIFT), and to explore potential causes of DRIFT. Methods, We studied ratings for 14 internal medicine resident doctors over the course of a single formative OSCE, comprising 10 12-minute stations, each with a single rater. We evaluated the association between time-slot and rating for a station. We also explored a possible interaction between time-slot and station difficulty, which would support the hypothesis that rater fatigue causes DRIFT, and considered ,warm-up' as an alternative explanation for DRIFT by repeating our analysis after excluding the first two OSCE stations. Results, Time-slot was positively associated with rating on a station (regression coefficient 0.88, 95% confidence interval [CI] 0.38,1.38; P = 0.001). There was an interaction between time-slot and station difficulty: for the more difficult stations the regression coefficient for time-slot was 1.24 (95% CI 0.55,1.93; P = 0.001) compared with 0.52 (95% CI , 0.08 to 1.13; P = 0.09) for the less difficult stations. Removing the first two stations from our analyses did not correct DRIFT. Conclusions, Systematic biases, such as DRIFT, may compromise internal validity in an OSCE. Further work is needed to confirm this finding and to explore whether DRIFT also affects ratings on summative OSCEs. If confirmed, the factors contributing to DRIFT, and ways to reduce these, should then be explored. [source] Bayesian galaxy shape measurement for weak lensing surveys , I. Methodology and a fast-fitting algorithmMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007L. Miller ABSTRACT The principles of measuring the shapes of galaxies by a model-fitting approach are discussed in the context of shape measurement for surveys of weak gravitational lensing. It is argued that such an approach should be optimal, allowing measurement with maximal signal-to-noise ratio, coupled with estimation of measurement errors. The distinction between likelihood-based and Bayesian methods is discussed. Systematic biases in the Bayesian method may be evaluated as part of the fitting process, and overall such an approach should yield unbiased shear estimation without requiring external calibration from simulations. The principal disadvantage of model fitting for large surveys is the computational time required, but here an algorithm is presented that enables large surveys to be analysed in feasible computation times. The method and algorithm is tested on simulated galaxies from the Shear TEsting Programme (STEP). [source] Systematic biases and culture in project failuresPROJECT MANAGEMENT JOURNAL, Issue 4 2008Barry Shore Abstract Project success rates have improved, and much of the credit can be given to the knowledge, practices, and standards that have contributed to the professionalization of the field. Unfortunately, too many failures still occur. Because many of them can be traced to management and decision-making practices, it might be useful at this stage to explore a set of systematic biases to determine if understanding them can help diagnose and perhaps even prevent failures from occurring. This article begins with a framework identifying the influences on project outcomes, defines the systematic biases that may derail projects, summarizes eight project failures, uses the framework to diagnose those failures, and concludes by suggesting how organizational and project culture may contribute to these very common and natural biases. [source] Methods to Test the Spatial Mismatch HypothesisECONOMIC GEOGRAPHY, Issue 4 2005Donald S. Houston Abstract: The spatial mismatch hypothesis postulates that employment deconcentration within U.S. metropolitan areas goes some way toward explaining higher unemployment and lower wages among ethnic minority groups, since these groups are more likely to reside in central-city areas. However, little consensus has emerged on the importance of spatial mismatch in explaining disadvantage in the labor market. This article argues that conflicting evidence is the result of the variety of methods that have been used to test the spatial mismatch hypothesis. Moreover, it draws attention to a number of hitherto uncovered flaws in some of these methods that introduce systematic biases against finding evidence in support of the hypothesis. In light of these flaws, favored methods for future research are highlighted. Drawing on evidence from British conurbations that display similar spatial inequalities to U.S. metropolitan areas despite much smaller ethnic minority populations, the article contends that race does not lie at the heart of the spatial mismatch problem. Three areas in which the spatial mismatch hypothesis should be reconceptualized are identified: first, its emphasis should be on spatial, not racial, inequalities; second, it needs to differentiate between residential immobility and residential segregation, which are quite different; and third, it needs to recognize that the extent and the effect of spatial mismatch are distinct and should be measured separately. [source] Fossilized embryos are widespread but the record is temporally and taxonomically biasedEVOLUTION AND DEVELOPMENT, Issue 2 2006Philip C. J. Donoghue SUMMARY We report new discoveries of embryos and egg capsules from the Lower Cambrian of Siberia, Middle Cambrian of Australia and Lower Ordovician of North America. Together with existing records, embryos have now been recorded from four of the seven continents. However, the new discoveries highlight secular and systematic biases in the fossil record of embryonic stages. The temporal window within which the embryos and egg capsules are found is of relatively short duration; it ends in the Early Ordovician and is roughly coincident with that of typical "Orsten"-type faunas. The reduced occurrence of such fossils has been attributed to reducing levels of phosphate in marine waters during the early Paleozoic, but may also be owing to the increasing depth of sediment mixing by infaunal metazoans. Furthermore, most records younger than the earliest Cambrian are of a single kind,large eggs and embryos of the priapulid-like scalidophoran Markuelia. We explore alternative explanations for the low taxonomic diversity of embryos recovered thus far, including sampling, size, anatomy, ecology, and environment, concluding that the preponderance of Markuelia embryos is due to its precocious development of cuticle at an embryonic stage, predisposing it to preservation through action as a substrate on which microbially mediated precipitation of authigenic calcium phosphate may occur. The fossil record of embryos may be limited to a late Neoproterozoic to early Ordovician snapshot that is subject to dramatic systematic bias. Together, these biases must be considered seriously in attempts to use the fossil record to arbitrate between hypotheses of developmental and life history evolution implicated in the origin of metazoan clades. [source] On the Use of Multifactor Models to Evaluate Mutual Fund PerformanceFINANCIAL MANAGEMENT, Issue 1 2009Joop Huij We show that multifactor performance estimates for mutual funds suffer from systematic biases and argue that these biases are a result of miscalculating the factor premiums. Because the factor proxies are based on hypothetical stock portfolios and do not incorporate transaction costs, trade impact, and trading restrictions, the factor premiums are either over- or underestimated. We argue that factor proxies based on mutual fund returns rather than on stock returns provide better benchmarks to evaluate professional money managers. [source] Tissue Oxygenation Does Not Predict Central Venous Oxygenation in Emergency Department Patients With Severe Sepsis and Septic ShockACADEMIC EMERGENCY MEDICINE, Issue 4 2010Anthony M. Napoli MD Abstract Objectives:, This study sought to determine whether tissue oxygenation (StO2) could be used as a surrogate for central venous oxygenation (ScVO2) in early goal-directed therapy (EGDT). Methods:, The study enrolled a prospective convenience sample of patients aged ,18 years with sepsis and systolic blood pressure <90 mm Hg after 2 L of normal saline or lactate >4 mmol, who received a continuous central venous oximetry catheter. StO2 and ScVO2 were measured at 15-minute intervals. Data were analyzed using a random coefficients model, correlations, and Bland-Altman plots. Results:, There were 284 measurements in 40 patients. While a statistically significant relationship existed between StO2 and ScVO2 (F(1,37) = 10.23, p = 0.002), StO2 appears to systematically overestimate at lower ScVO2 and underestimate at higher ScVO2. This was reflected in the fixed effect slope of 0.49 (95% confidence interval [CI] = 0.266 to 0.720) and intercept of 34 (95% CI = 14.681 to 50.830), which were significantly different from 1 and 0, respectively. The initial point correlation (r = 0.5) was fair, but there was poor overall agreement (bias = 4.3, limits of agreement = ,20.8 to 29.4). Conclusions:, Correlation between StO2 and ScVO2 was fair. The two measures trend in the same direction, but clinical use of StO2 in lieu of ScVO2 is unsubstantiated due to large and systematic biases. However, these biases may reflect real physiologic states. Further research may investigate if these measures could be used in concert as prognostic indicators. ACADEMIC EMERGENCY MEDICINE 2010; 17:349,352 © 2010 by the Society for Academic Emergency Medicine [source] Net primary productivity mapped for Canada at 1-km resolutionGLOBAL ECOLOGY, Issue 2 2002J Liu Abstract Aim To map net primary productivity (NPP) over the Canadian landmass at 1-km resolution. Location Canada. Methods A simulation model, the Boreal Ecosystem Productivity Simulator (BEPS), has been developed. The model uses a sunlit and shaded leaf separation strategy and a daily integration scheme in order to implement an instantaneous leaf-level photosynthesis model over large areas. Two key driving variables, leaf area index (every 10 days) and land cover type (annual), are derived from satellite measurements of the Advanced Very High Resolution Radiometer (AVHRR). Other spatially explicit input data are also prepared, including daily meteorological data (radiation, precipitation, temperature, and humidity), available soil water holding capacity (AWC) and forest biomass. The model outputs are compared with ground plot data to ensure that no significant systematic biases are created. Results The simulation results show that Canada's annual net primary production was 1.22 Gt C year,1 in 1994, 78% attributed to forests, mainly the boreal forest, without considering the contribution of the understorey. The NPP averaged over the entire landmass was ~140 g C m,2 year,1 in 1994. Geographically, NPP varied greatly among ecozones and provinces/territories. The seasonality of NPP is characterized by strong summer photosynthesis capacities and a short growing season in northern ecosystems. Conclusions This study is the first attempt to simulate Canada-wide NPP with a process-based model at 1-km resolution and using a daily step. The statistics of NPP are therefore expected to be more accurate than previous analyses at coarser spatial or temporal resolutions. The use of remote sensing data makes such simulations possible. BEPS is capable of integrating the effects of climate, vegetation, and soil on plant growth at a regional scale. BEPS and its parameterization scheme and products can be a basis for future studies of the carbon cycle in mid-high latitude ecosystems. [source] CREATING VALUE IN PENSION PLANS (OR, GENTLEMEN PREFER BONDS)JOURNAL OF APPLIED CORPORATE FINANCE, Issue 4 2003Jeremy Gold Pension funds are typically one-half to two-thirds invested in equities because equities are expected to outperform other financial assets over the long term, and the long-term nature of pension fund liabilities seems well suited to absorbing any short-term return volatility. What's more, U.S. GAAP currently makes it possible to take credit in advance for the higher anticipated earnings on equity investments without acknowledging their inherent risk. But by allowing the higher expected returns from stocks to reduce a company's current pension expenses, the accounting treatment conflicts with some very basic principles of finance (in particular, the idea that investors must earn higher returns on riskier investments just to "break even"), conceals systematic biases in the actuarial analysis, and gives managers considerable latitude to manipulate the bottom line. The authors suggest a startlingly different approach. They argue that pension assets should be invested entirely in duration-matched debt instruments for two reasons: (1) to capture the full tax benefits of pre-funding their pension obligations and (2) to improve overall corporate risk profiles by converting general stock market risk into firm-specific operating risk, where corporate managers should have a comparative advantage and can generate real value. Investing exclusively in bonds would take better advantage of the tax-exempt status of pension plans and greatly reduce fund management costs, while at the same time helping o shore up fund quality and sharpening corporate executives' focus on their real operating assets. [source] Age and probabilistic reasoning: Biases in conjunctive, disjunctive and Bayesian judgements in early and late adulthoodJOURNAL OF BEHAVIORAL DECISION MAKING, Issue 1 2005John E. Fisk Abstract Probabilistic reasoning plays an essential part in many aspects of our daily routine and it has been argued that as we grow older, the need to make judgements under uncertainty becomes increasingly important. Two studies were conducted to establish whether the propensity to commit probabilistic reasoning errors increased with age. Young (aged 16,24), middle aged (25,54), and older persons (55 years and above) were included. Study 1 revealed systematic biases and errors across a range of judgement tasks. However, no evidence of any age effect in Bayesian inference, the incidence of the conjunction fallacy, or in the number of disjunction errors was found. The results obtained in Study 1 were replicated in Study 2, where the potential mediating role of working memory processes and intellectual capacity were explicitly assessed. While some aspects of probabilistic reasoning performance were correlated with measures of intelligence and working memory functioning among young adults, this was much less evident in older persons. The present findings are discussed in relation to the evolution of the dualistic heuristic,analytical system over the adult lifespan. Copyright © 2005 John Wiley & Sons, Ltd. [source] The effect of differential rater function over time (DRIFT) on objective structured clinical examination ratingsMEDICAL EDUCATION, Issue 10 2009Kevin McLaughlin Context, Despite the impartiality implied in its title, the objective structured clinical examination (OSCE) is vulnerable to systematic biases, particularly those affecting raters' performance. In this study our aim was to examine OSCE ratings for evidence of differential rater function over time (DRIFT), and to explore potential causes of DRIFT. Methods, We studied ratings for 14 internal medicine resident doctors over the course of a single formative OSCE, comprising 10 12-minute stations, each with a single rater. We evaluated the association between time-slot and rating for a station. We also explored a possible interaction between time-slot and station difficulty, which would support the hypothesis that rater fatigue causes DRIFT, and considered ,warm-up' as an alternative explanation for DRIFT by repeating our analysis after excluding the first two OSCE stations. Results, Time-slot was positively associated with rating on a station (regression coefficient 0.88, 95% confidence interval [CI] 0.38,1.38; P = 0.001). There was an interaction between time-slot and station difficulty: for the more difficult stations the regression coefficient for time-slot was 1.24 (95% CI 0.55,1.93; P = 0.001) compared with 0.52 (95% CI , 0.08 to 1.13; P = 0.09) for the less difficult stations. Removing the first two stations from our analyses did not correct DRIFT. Conclusions, Systematic biases, such as DRIFT, may compromise internal validity in an OSCE. Further work is needed to confirm this finding and to explore whether DRIFT also affects ratings on summative OSCEs. If confirmed, the factors contributing to DRIFT, and ways to reduce these, should then be explored. [source] Revisiting two local constraints of the Galactic chemical evolutionMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006M. Haywood ABSTRACT I review the uncertainties in two observational local constraints of the Galactic disc chemical evolution: the metallicity distribution of long-lived dwarfs and the age,metallicity relation. Analysing most recent data, it is shown first that the observed metallicity distribution at solar galactocentric radius, designed with standard methods, is more fit to a closed-box model than to the infall metallicity distribution. We argue that this is due to the specific contribution of the thick-disc population, which has been overlooked both in the derivation of the observed metallicity distribution and in the standard chemical evolution models. Although this agreement disqualifies the metallicity distribution as the best supportive (indirect) evidence for infall, we argue that the evolution must be more complex than described by either the closed-box or the standard infall models. It is then shown that recent determinations of the age,metallicity distribution (AMD) from large Strömgren photometric surveys are dominated by noise resulting from systematic biases in metallicities and effective temperatures. These biases are evaluated and a new AMD is obtained, where particularities of the previous determinations are phased out. The new age,metallicity relation shows a mean increase limited to about a factor of 2 in Z over the disc age. It is shown that below 3 Gyr, the dispersion in metallicity is about 0.1 dex, which, given the observational uncertainties in the derived metallicities, is compatible with the small cosmic dispersion measured on the interstellar medium and meteoritic pre-solar dust grains. A population that is progressively older and more metal rich arises at a metallicity greater than that of the Hyades, to reach [Fe/H],+0.5 dex at ages greater than 5 Gyr. We suggest that this is best explained by radial migration. A symmetrical widening of the metallicity interval towards lower values is seen at about the same age, which is attributed to a similar cause. Finally, the new derived ages are sufficiently consistent that an age,metallicity relation within the thick disc is confirmed. These new features altogether draw a picture of the chemical evolution in the solar neighbourhood where dynamical effects and complexity in the AMD dominate, rather than a generalized high dispersion at all ages. [source] Isochrone ages for field dwarfs: method and application to the age,metallicity relationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004Frédéric Pont ABSTRACT A new method is presented to compute age estimates from theoretical isochrones using temperature, luminosity and metallicity data for individual stars. Based on Bayesian probability theory, this method avoids the systematic biases affecting simpler strategies and provides reliable estimates of the age probability distribution function for late-type dwarfs. Basic assumptions concerning the a priori parameter distribution suitable for the solar neighbourhood are combined with the likelihood assigned to the observed data to yield the complete posterior age probability. This method is especially relevant for G dwarfs in the 3,15 Gyr range of ages, crucial to the study of the chemical and dynamical history of the Galaxy. In many cases, it yields markedly different results from the traditional approach of reading the derived age from the isochrone nearest to the data point. We show that the strongest process affecting the traditional approach is that of strongly favouring computed ages near the end-of-main-sequence lifetime. The Bayesian method compensates for this potential bias and generally assigns much higher probabilities to lower main-sequence ages, compared with short-lived evolved stages. This has a strong influence on any application to galactic studies, especially given the present uncertainties on the absolute temperature scale of the stellar evolution models. In particular, the known mismatch between the model predictions and the observations for moderately metal-poor dwarfs (,1 < [Fe/H] < ,0.3) has a dramatic effect on the traditional age determination. We apply our method to the classic sample of Edvardsson et al., who derived the age,metallicity relation (AMR) of 189 field dwarfs with precisely determined abundances. We show how much of the observed scatter in the AMR is caused by the interplay between the systematic biases affecting the traditional age determination, the colour mismatch with the evolution models and the presence of undetected binaries. Using new parallax, temperature and metallicity data, our age determination for the same sample indicates that the intrinsic dispersion in the AMR is at most 0.15 dex and probably lower. In particular, we show that old, metal-rich objects ([Fe/H], 0.0 dex, age > 5 Gyr) and young, metal-poor objects ([Fe/H] < ,0.5 dex, age < 6 Gyr) in many observed AMR plots are artefacts caused by too simple a treatment of the age determination. The incompatibility of those AMR plots with a well-mixed interstellar medium may therefore only be apparent. Incidentally, our results tend to restore confidence in the method of age determination from the chromospheric activity for field dwarfs. [source] Systematic biases and culture in project failuresPROJECT MANAGEMENT JOURNAL, Issue 4 2008Barry Shore Abstract Project success rates have improved, and much of the credit can be given to the knowledge, practices, and standards that have contributed to the professionalization of the field. Unfortunately, too many failures still occur. Because many of them can be traced to management and decision-making practices, it might be useful at this stage to explore a set of systematic biases to determine if understanding them can help diagnose and perhaps even prevent failures from occurring. This article begins with a framework identifying the influences on project outcomes, defines the systematic biases that may derail projects, summarizes eight project failures, uses the framework to diagnose those failures, and concludes by suggesting how organizational and project culture may contribute to these very common and natural biases. [source] The role of the basic state in the ENSO,monsoon relationship and implications for predictabilityTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 607 2005A. G. Turner Abstract The impact of systematic model errors on a coupled simulation of the Asian summer monsoon and its interannual variability is studied. Although the mean monsoon climate is reasonably well captured, systematic errors in the equatorial Pacific mean that the monsoon,ENSO teleconnection is rather poorly represented in the general-circulation model. A system of ocean-surface heat flux adjustments is implemented in the tropical Pacific and Indian Oceans in order to reduce the systematic biases. In this version of the general-circulation model, the monsoon,ENSO teleconnection is better simulated, particularly the lag,lead relationships in which weak monsoons precede the peak of El Niño. In part this is related to changes in the characteristics of El Niño, which has a more realistic evolution in its developing phase. A stronger ENSO amplitude in the new model version also feeds back to further strengthen the teleconnection. These results have important implications for the use of coupled models for seasonal prediction of systems such as the monsoon, and suggest that some form of flux correction may have significant benefits where model systematic error compromises important teleconnections and modes of interannual variability. Copyright © 2005 Royal Meteorological Society [source] The sheep genome reference sequence: a work in progressANIMAL GENETICS, Issue 5 2010The International Sheep Genomics Consortium Summary Until recently, the construction of a reference genome was performed using Sanger sequencing alone. The emergence of next-generation sequencing platforms now means reference genomes may incorporate sequence data generated from a range of sequencing platforms, each of which have different read length, systematic biases and mate-pair characteristics. The objective of this review is to inform the mammalian genomics community about the experimental strategy being pursued by the International Sheep Genomics Consortium (ISGC) to construct the draft reference genome of sheep (Ovis aries). Component activities such as data generation, sequence assembly and annotation are described, along with information concerning the key researchers performing the work. This aims to foster future participation from across the research community through the coordinated activities of the consortium. The review also serves as a ,marker paper' by providing information concerning the pre-publication release of the reference genome. This ensures the ISGC adheres to the framework for data sharing established at the recent Toronto International Data Release Workshop and provides guidelines for data users. [source] Toward an unbiased sample of X-ray selected normal galaxies outside the local UniverseASTRONOMISCHE NACHRICHTEN, Issue 2 2008A. GeorgakakisArticle first published online: 14 FEB 200 Abstract This paper shows that our understanding of the statistical properties of X-ray selected normal galaxies (e.g. X-ray luminosity function) can be significantly improved by combining a wide-area XMM-Newton survey with the moderare resolution and high S/N optical spectroscopy of the SDSS. Such a combined dataset has the potential to minimise uncertainties that affect existing normal galaxy samples at X-rays, such as small number statistics, cosmic variance, AGN contamination and incompleteness at bright X-ray luminosities. It is demonstrated that a 100 deg2 XMM-Newton survey in the SDSS area to the limit fX(0.5,2 keV) , 5 × 10,15 erg cm,2 s,1 will detect over 400 X-ray selected normal galaxies with excellent control over systematic biases, thereby providing tight contraints on the X-ray luminosity function at z , 0.1. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Summary of recommendations of the first workshop on Postprocessing and Downscaling Atmospheric Forecasts for Hydrologic Applications held at Météo-France, Toulouse, France, 15,18 June 2009ATMOSPHERIC SCIENCE LETTERS, Issue 2 2010John Schaake Abstract Hydrologists are increasingly using numerical weather forecasting products as an input to their hydrological models. These products are often generated on relatively coarse scales compared with hydrologically relevant basin units and suffer systematic biases that may have considerable impact when passed through the nonlinear hydrological filters. Therefore, the data need processing before they can be used in hydrological applications. This manuscript summarises discussions and recommendations of the first workshop on Postprocessing and Downscaling Atmospheric Forecasts for Hydrologic Applications held at Meteo France, Toulouse, France, 15,18 June 2008. The recommendations were developed by work groups that considered the following three areas of ensemble prediction: (1) short range (0,2 days), (2) medium range (3 days to 2 weeks), and (3) sub-seasonal and seasonal (beyond 2 weeks). Copyright © 2010 Royal Meteorological Society [source] | ||||||||||||||||||||||