ISSN:0277-6715    

DOI:10.1002/sim.4780111002

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractIn epidemiologic studies, two forms of collinear relationships between the intake of major nutrients, high correlations, and the relative homogeneity of the diet, can yield unstable and not easily interpreted regression estimates for the effect of diet on disease risk. This paper presents tools for assessing the magnitude and source of the corresponding collinear relationships among the estimated coefficients for relative risk regression models. I show how to extend three tools (condition indices, variance decomposition proportions, and standard inflation factors) for diagnosing collinearity in standard regression models to likelihood and partial likelihood estimation for logistic and proportional hazards models. This extension is based on the analogue role of the information matrix in such analyses and the cross‐product matrix in the standard linear model. I apply the methodology to relative risk models that relate crude intakes (on the log scale) and nutrient densities to breast cancer cases in the NHANES‐I follow‐up study. The three diagnostic tools provide complementary evidence of the existence of a strong collinearity in all models that is due largely to homogeneity of the population with respect to our risk scale for the crude intakes. The analysis suggests that the non‐significant relative risks for the crude intakes in these models may be due to their involvement in collinear relationships, while the nonsignificant relative risks for the nutrient densities are far less affected by multicolli

ISSN:0277-6715    

DOI:10.1002/sim.4780111003

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractThis paper considers a statistic — recently suggested by Mora — for the deviation of a sample distribution from a reference distribution which typically arises in anthropometry when using the nutritional indicators height/age, weight/age or weight/height. The statistic measures the area between curves (ABC) and stands for the mass of the sample distribution which is not covered by the reference distribution. The paper provides a statistical framework for the ABC and includes some minor corrections of Mora's original paper. For the normal distribution situation with common or different variances, formulae are derived which include a partition of ABC into parts corresponding to malnourished and well‐nourished groups. However, themainresult is a non‐parametric generalization of the ABC, motivated by the fact that the nutritional indicators often have skewed distributions with heavier left tails. Non‐parametric statistical inference is provided by linking the ABC to the Kolmogorov‐Smirno

ISSN:0277-6715    

DOI:10.1002/sim.4780111004

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractReference centile curves show the distribution of a measurement as it changes according to some covariate, often age. The LMS method summarizes the changing distribution by three curves representing the median, coefficient of variation and skewness, the latter expressed as a Box‐Cox power. Using penalized likelihood the three curves can be fitted as cubic splines by non‐linear regression, and the extent of smoothing required can be expressed in terms of smoothing parameters or equivalent degrees of freedom. The method is illustrated with data on triceps skinfold in Gambian girls and women, and body weight in U.S.A. gi

ISSN:0277-6715    

DOI:10.1002/sim.4780111005

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractSince 1983, hospitals in the United States have been receiving prospective payment for their in‐hospital patient admissions covered under Medicare. Under such schemes each patient is placed in a group by a classification system, known as the Diagnosis Related Groups (DRG), and the hospital is reimbursed by the Health Care Financing Administration according to some predetermined group average, adjusted for hospital level characteristics, such as size, location and teaching activity. Recent interest has focused on refining the DRG system or considering totally different systems of classification. Studies designed to compare the ability of different systems to account for between‐patient variability in resource consumption in the same dataset lead to the problem of model selection between large non‐nested regressions, where resource consumption, measured by length of hospital stay or costs, is regressed on dummy‐indicator variables representing different patient groups. We use a simple measure of fit to develop a symmetric test of the null hypothesis that the two systems account equally well for variability in resource consumption. With this method, unlike methods such as Akaike's AIC criterion, we can quantify the probability of a false positive, and thereby limit the probability of choosing one system over another when it is no better at accounting for variability in resource cons

ISSN:0277-6715    

DOI:10.1002/sim.4780111006

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractThe 75/75 rule was originally proposed by the U.S. Food and Drug Administration (FDA) as an alternative means of testing the bioequivalence of two formulations of a pharmaceutical agent. The rule specified that the ratio of test‐to‐reference formulation of a bioavailability measure arising in a bioequivalence study must be between 75 and 125 per cent of unity in at least 75 per cent of subjects to declare two formulations bioequivalent. The rule has garnered criticism in the literature and the FDA no longer uses the rule formally in assessing bioequivalence. The basis, however, for all criticism of the rule has been simulation arguments. In this paper, we derive the sampling model implied by the rule and place the rule in the framework of a statistical hypothesis test. We show how the significance level of the test depends upon variability of the formulations, and thus why the rule performs in the way that has received critic

ISSN:0277-6715    

DOI:10.1002/sim.4780111007

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractMany clinical, environmental, and epidemiologic studies rely heavily upon biochemical data, and the quality of these data is of paramount importance to the validity of study conclusions. Traditionally, far more attention has been given to the analysis of study data than has been given to monitoring the quality of the data. In this paper we draw an analogy between monitoring a laboratory system and an industrial production process and discuss the limitations of industrial quality control plans when applied in a laboratory setting. We derive methods for computing optimal checking schedules for laboratory analyses. These schedules formalize traditional laboratory practices of periodic checking and provide guidelines for the frequency and placement of checks within a finite batch of analyses. When laboratory system failure can be reasonably approximated by an exponential or geometric distribution, optimal checking schedules are relatively easy to compute. For more complex failure distributions, we present a dynamic programming approach. We describe an application to the measurement of selenium status in plasma samples using an electrothermal atomic absorption spectrophotometry procedure.

ISSN:0277-6715    

DOI:10.1002/sim.4780111008

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractThe effects of withdrawal censoring on the power of the group sequential logrank procedure are examined in the context of survival studies. Special attention is given to an assumption that the withdrawal censoring mechanism is only conditionally independent of survival time. Simulation studies of two‐sample, fixed duration trials, under random right censoring, staggered entry and exponential life times, reveal a general decrease in statistical power that is most pronounced when the withdrawals are unbalanced, the majority occurring in the group with the higher hazard rate. Generally, this decrease in power is not of practical concern under typical withdrawal rates except when conditional independence is present in which case large fluctuations in power and type I error rates can occu

ISSN:0277-6715    

DOI:10.1002/sim.4780111009

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractThe Southwest Oncology Group (SWOG) is a multicentre co‐operative group that performs prospective clinical trials in cancer. The SWOG Statistical Center is responsible for preparing a report of studies for the semiannual meetings. The report includes a summary of study design and eligibility criteria, current accrual, baseline patient characteristics, toxicity, and, for studies closed to accrual and with sufficiently mature outcome data, tumour response or patient survival. A software program, Statisticians' Report Worksheet (SRW), has been developed by the programming staff at the SWOG Statistical Center. This program enables each disease specific chapter to be created independently, but ensures that tables and graphs for all chapters are of uniform style. We describe here the coordination of the report, and the use of the SRW program. This reporting system can serve as a model for other group

ISSN:0277-6715    

DOI:10.1002/sim.4780111010

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractPhase I clinical trials are conducted to determine the dose‐response curve of a new drug with respect to toxic side effects and, in particular, to estimate the maximum tolerated dose (MTD). In this paper we take a Bayesian approach to the problem of making inferences about the MTD. Working with broad classes of priors, we obtain the posterior distribution of the MTD and study its properties. We also address the question of providing updated assessments of the risk of toxicity for new patients entering the study at a specific dose level. These assessments would be useful in deciding issues of study management and ethics. Our analysis pays particular attention to the sensitivity of the inferences and risk assessments to the choice of prior and the choice of model for the dose‐response relations

ISSN:0277-6715    

DOI:10.1002/sim.4780111011

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY