Home About us Contact
|
Home About us Contact | ||
|
|
||
Statistical Assumptions (statistical + assumption)
Selected AbstractsChallenges and regulatory experiences with non-inferiority trial design without placebo armBIOMETRICAL JOURNAL, Issue 2 2009H. M. James Hung Abstract For a non-inferiority trial without a placebo arm, the direct comparison between the test treatment and the selected positive control is in principle the only basis for statistical inference. Therefore, evaluating the test treatment relative to the non-existent placebo presents extreme challenges and requires some kind of bridging from the past to the present with no current placebo data. For such inference based partly on an indirect bridging manipulation, fixed margin method and synthesis method are the two widely discussed methods in the recent literature. There are major differences in statistical inference paradigm between the two methods. The fixed margin method employs the historical data that assess the performances of the active control versus a placebo to guide the selection of the non-inferiority margin. Such guidance is not part of the ultimate statistical inference in the non-inferiority trial. In contrast, the synthesis method connects the historical data to the non-inferiority trial data for making broader inferences relating the test treatment to the non-existent current placebo. On the other hand, the type I error rate associated with the direct comparison between the test treatment and the active control cannot shed any light on the appropriateness of the indirect inference for faring the test treatment against the non-existent placebo. This work explores an approach for assessing the impact of potential bias due to violation of a key statistical assumption to guide determination of the non-inferiority margin. [source] Marketing Category Forecasting: An Alternative of BVAR-Artificial Neural Networks¶DECISION SCIENCES, Issue 4 2000James J. Jiang ABSTRACT Analyzing scanner data in brand management activities presents unique difficulties due to the vast quantity of the data. Time series methods that are able to handle the volume effectively often are inappropriate due to the violation of many statistical assumptions in the data characteristics. We examine scanner data sets for three brand categories and examine properties associated with many time series forecasting methods. Many violations are found with respect to linearity, normality, autocorrelation, and heteroscedasticity. With this in mind we compare the forecasting ability of neural networks that require no assumptions to two of the more robust time series techniques. Neural networks provide similar forecasts to Bayesian vector autoregression (BVAR), and both outperform generalized autoregressive conditional herteroscedasticty (GARCH) models. [source] Incidence of inflammatory bowel disease in finnish children, 1987,2003INFLAMMATORY BOWEL DISEASES, Issue 8 2006Pieta Turunen MS Abstract Background: The incidence of inflammatory bowel disease (IBD) has been increasing in Western countries. In younger people, Crohn's disease (CD) predominates over ulcerative colitis (UC), but the finding is not universal. The present study aimed to characterize not only the incidence but also the clinical picture of IBD from 1987 to 2003 in a large pediatric population in Finland. Materials and Methods: Data were collected from the patient discharge and medical records at the 2 largest university hospitals in Finland. The study population covered a total of 619,340 children, representing 56% of the children <18 years old in the country. All of the cases diagnosed with IBD from 1987 to 2003 were reviewed. Clinical, endoscopic, and histological data were collected. Incidence rates were estimated based on statistical assumptions. Results: A total of 604 cases with IBD were diagnosed during the 17-year period. All of the patients had undergone endoscopy. The diagnosis was CD in 203 (34%) cases, UC in 317 (52%) cases, and indeterminate colitis (IC) in 83 (14%) cases. The mean annual incidence rate increased from 3.9/100,000 (95% confidence interval [CI] 2.5,5.8) in 1987 to 7.0/100,000 (CI 5.0,9.4) in 2003 (P < 0.001). The majority of cases were 12 to <15 years old (n = 200, 33%). Of the patients, 5.1% were <3 years old and 14% were <6 years old. IC was most common in young children; 29% of all IBD patients <3 years of age had IC. Of the patients, 97% had been followed up until the age 18 in the hospitals after initial diagnosis (median follow-up 3.1 years). Of the patients, 45.2% were initially treated with steroids, whereas 17.8% received immunosuppressive agents at the end of the follow-up. Operations had been performed in 21% of the cases before age 18. The median time interval from the diagnosis to the first operation was 1.8 (range 7.8) years. Conclusions: The incidence of pediatric IBD almost doubled in Finland from 1987 to 2003. Surgical intervention was common early in the disease course. [source] Relation between macroscopic and microscopic dielectric relaxation times in water dynamicsISRAEL JOURNAL OF CHEMISTRY, Issue 3-4 2003Vladimir I. Arkhipov A simplified derivation for the ratio of macroscopic to microscopic relaxation times of polar liquids is based on the Mori-Zwanzig projection-operator technique, with added statistical assumptions. We obtain several useful forms for the lifetime ratio, which we apply to the dynamics of liquid water. Our theoretical single-molecule relaxation times agree with the second Debye relaxation times as measured by frequency-domain dielectric spectroscopy of water and alcohols. From the theory, fast relaxation modes couple to the Debye relaxation time, ,d, through very large water clusters, and their temperature dependence is similar to that of ,d. Slower modes are localized to smaller water clusters and exhibit weaker temperature dependence. This is exemplified by the lifetime ratios measured by time-domain dielectric spectroscopy and optical Kerr effect spectroscopy, respectively. [source] The wolves of Isle Royale display scale-invariant satiation and ratio-dependent predation on mooseJOURNAL OF ANIMAL ECOLOGY, Issue 5 2005CHRISTIAN JOST Summary 1The importance of two features of the predator functional response (satiation and predator dependence) is investigated in the wolf,moose interaction on Isle Royale National Park (Michigan, USA). This is done by fitting and comparing nine different functional response models to the observed kill rates. 2Three different observational scales (the whole island, the wolf packs, or a ,mixed' scale) are used to assess the sensitivity of the detected properties with respect to these spatial scales. 3Independently of the observational scale and of statistical assumptions on data structure, strong predator dependence and satiation of the wolf functional response are found. The ,mixed' scale gives the most consistent results, suggesting that predation should be measured for each pack, but that packs share all moose on the island. On this scale, the functional response is clearly ratio-dependent. [source] Maximum likelihood fitting using ordinary least squares algorithms,JOURNAL OF CHEMOMETRICS, Issue 8-10 2002Rasmus Bro Abstract In this paper a general algorithm is provided for maximum likelihood fitting of deterministic models subject to Gaussian-distributed residual variation (including any type of non-singular covariance). By deterministic models is meant models in which no distributional assumptions are valid (or applied) on the parameters. The algorithm may also more generally be used for weighted least squares (WLS) fitting in situations where either distributional assumptions are not available or other than statistical assumptions guide the choice of loss function. The algorithm to solve the associated problem is called MILES (Maximum likelihood via Iterative Least squares EStimation). It is shown that the sought parameters can be estimated using simple least squares (LS) algorithms in an iterative fashion. The algorithm is based on iterative majorization and extends earlier work for WLS fitting of models with heteroscedastic uncorrelated residual variation. The algorithm is shown to include several current algorithms as special cases. For example, maximum likelihood principal component analysis models with and without offsets can be easily fitted with MILES. The MILES algorithm is simple and can be implemented as an outer loop in any least squares algorithm, e.g. for analysis of variance, regression, response surface modeling, etc. Several examples are provided on the use of MILES. Copyright © 2002 John Wiley & Sons, Ltd. [source] Item response theory: applications of modern test theory in medical educationMEDICAL EDUCATION, Issue 8 2003Steven M Downing Context Item response theory (IRT) measurement models are discussed in the context of their potential usefulness in various medical education settings such as assessment of achievement and evaluation of clinical performance. Purpose The purpose of this article is to compare and contrast IRT measurement with the more familiar classical measurement theory (CMT) and to explore the benefits of IRT applications in typical medical education settings. Summary CMT, the more common measurement model used in medical education, is straightforward and intuitive. Its limitation is that it is sample-dependent, in that all statistics are confounded with the particular sample of examinees who completed the assessment. Examinee scores from IRT are independent of the particular sample of test questions or assessment stimuli. Also, item characteristics, such as item difficulty, are independent of the particular sample of examinees. The IRT characteristic of invariance permits easy equating of examination scores, which places scores on a constant measurement scale and permits the legitimate comparison of student ability change over time. Three common IRT models and their statistical assumptions are discussed. IRT applications in computer-adaptive testing and as a method useful for adjusting rater error in clinical performance assessments are overviewed. Conclusions IRT measurement is a powerful tool used to solve a major problem of CMT, that is, the confounding of examinee ability with item characteristics. IRT measurement addresses important issues in medical education, such as eliminating rater error from performance assessments. [source] Wage differentials and state-private sector employment choice in Yugoslavia*THE ECONOMICS OF TRANSITION, Issue 3 2003Michael M. Lokshin Abstract In this study we use the newly available Yugoslavian Labor Force Survey data to investigate wage differentials and employment decisions in the state and private sectors in Yugoslavia. For the analysis we use three empirical models that rely on different statistical assumptions. We extend the standard switching regression model to allow non-normality in the joint distribution of the error terms. After correcting for the sector selection bias and controlling for workers' characteristics we find a private sector wage advantage. The wage premium is largest for workers with low education levels and declining for workers with higher educational levels. Given the regulatory and tax policies that pushed the private sector into the informal sphere of the economy during the period covered by our data, we argue that the state/private wage gap is likely to grow in the future. This will make it increasingly difficult for the state sector to attract and retain highly skilled employees. [source] Assessment of Multiple Ordinal EndpointsBIOMETRICAL JOURNAL, Issue 1 2009Lothar Häberle Abstract Ranking multivariate ordinal data and applying a non-parametric test is an analytical approach commonly employed to compare treatments. We study three types of ranking and demonstrate how to combine them. The ranking methods rest upon partial orders of the multidimensional measurements or upon the sum of ranks. Since their usage is simple as regards statistical assumptions and technical realization, they are also adapted for health professionals without deep statistical knowledge. Our goal is discussing differences between the approaches and disclosing possible statistical consequences of their usage (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||