摘要:

AbstractThis paper focuses on the effect of estimating the calibration parameters on the decision and detection limits for linear homoscedastic assays. Assay decision and detection limits are defined. For linear homoscedastic assays expressions for these limits are derived when the calibration parameters are known and unknwon. These expressions, together with others previously reported in the literature, are used to calculate limits for a particular example of a cyclic AMP assay. The various methods of computing decision and detection limits are compared in a simulation study. The results demonstrate the importance of taking into consideration the variation in the estimators of the calibration parameters.

ISSN:0277-6715    

DOI:10.1002/sim.4780141802

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

年代:1995

数据来源: WILEY

摘要:

AbstractFor many clinical trials and epidemiologic investigations in the field of ophthalmology, paired ordinal data are often collected through the detailed grading of retinal photographs. One method for analysis of these data is the extension of the generalized estimating equation (GEE) methodology to multinomial data with cumulative link functions. Prior to the development of this advanced technique, however, ophthalmologists developed a method of combining the ordinal responses of both eyes of a patient into a single person‐level response on a new ordinal scale. A relationship between the regression coefficients of these two methods is derived as a function of the correlation between eyes. We investigate the applicability of this result and the relationship of the standard errors in simulation experiments and in an example from the Wisconsin Epidemiologic Study of Diabetic Retinopath

ISSN:0277-6715    

DOI:10.1002/sim.4780141803

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

年代:1995

数据来源: WILEY

摘要:

AbstractThis paper discusses the application of a multi‐state model to diabetic retinopathy under the assumption that a continuous time Markov process determines the transition times between disease stages. The multi‐state model consists of three transient states that represent the early stages of retinopathy, and one final absorbing state that represent the irreversible stage of retinopathy. By using a model with covariables, we explore the effects of factors that influence the onset, progression, and regression of diabetic retinopathy among subjects with insulin‐dependent diabetes mellitus. We can also introduce time‐dependent covariables in the model by assuming that the covariables remain constant between two observations. We can also obtain survival‐type curves from each stage of the disease and for any combination of patient ris

ISSN:0277-6715    

DOI:10.1002/sim.4780141804

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

年代:1995

数据来源: WILEY

摘要:

AbstractFor time to event data with many potential failure types, one cannot uniquely determine the distribution of time to a specific event type, or marginal survival distribution, in the case where event types are mutually exclusive. In this paper we discuss several methods for estimating functions that bound the non‐identifiable marginal survival distribution in the competing risks problem. We compute and compare bounds for data simulated from two bivariate survival distributions. Results show that the methods provide a suitable estimate of the marginal survival probability when one has specified dependence correctly. Data from a large clinical trial for breast cancer illustrate the method

ISSN:0277-6715    

DOI:10.1002/sim.4780141805

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

年代:1995

数据来源: WILEY

摘要:

AbstractThe construction, validation and updating of a prognostic model for kidney graft survival is reported using data from the Eurotransplant database. First, a model is constructed for data from transplantations in the period 1984 to 1987. The model is later updated for the 1988–1990 data. The first data set was randomly split into a training set (two‐thirds of the data) and a validation set (one‐third). To prevent overfitting empirical Bayes estimation of the transplantation centre effect was employed. After that, the validation set was used for fine‐tuning by shrinkage. For updating with the 1988–1990 data parametric models were used after suitable transformation of the time axis; it appeared that survival had slightly improved. This necessitated a correction of the parameters in the exponential model. Correctness of the model was checked by extension to a Weibull model. The lack of fit was statistically significant, but practically ignorable. Recommendations are made to place less emphasis on the selection of variables and cut‐off points, and more emphasis on the fine‐tuning of the prognostic model by means of low‐dimensional parametric models in indepe

ISSN:0277-6715    

DOI:10.1002/sim.4780141806

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

年代:1995

数据来源: WILEY

摘要:

AbstractThe shape invariant model is a semi‐parametric approach to estimating a function relationship from clustered data (multiple observations on each of a number of individuals). The common response curve shape over individuals is estimated by adjusting for individual scaling differences while pooling shape information. In practice, the common response curve is restricted to some flexible family of functions. This paper introduces the use of a free‐knot spline shape function and reduces the number of parameters in the shape invariant model by assuming a random distribution on the parameters that control the individual scaling of the shape function. New graphical diagnostics are presented, parameter identifiability and estimation are discussed, and an example is presen

ISSN:0277-6715    

DOI:10.1002/sim.4780141807

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

年代:1995

数据来源: WILEY

摘要:

AbstractTo be consistent, censored data linear regression estimators typically require a correctly specified linear regression function and independent and identically distributed errors. For uncensored data one can assess these model assumptions informally by examining plots of the residuals against the independent variables or fitted values. In this paper I propose plots for censored data analogous to these uncensored data residual plots. One can use such plots in the same way as their uncensored data counterparts for checking model assumptions; if the model assumptions are correct, then the plots should exhibit a random scatter. I show that the proposed plots are useful in selecting a linear regression model for the Stanford heart transplant data.

ISSN:0277-6715    

DOI:10.1002/sim.4780141808

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

年代:1995

数据来源: WILEY

摘要:

AbstractWhen designing a trial to establish that a new treatment is as effective as a standard one, the conventional test procedure and sample size based on a null hypothesis of no difference between two treatments is inappropriate. Several authors have investigated test statistics and corresponding sample sizes based on the null hypothesis that the standard treatment is more effective than the new by at least some specific value for a single 2 × 2 table. This paper considers a trial that involves several 2 × 2 tables and presents an approximate formula for the sample size required to obtain a given power of a one‐tailed score test for a null hypothesis of a specific common non‐zero difference between two treatments across strata. I show that the sample size for a trial based on an unstratified test is always larger than that based on a stratified test when the design is bal

ISSN:0277-6715    

DOI:10.1002/sim.4780141809

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

年代:1995

数据来源: WILEY

摘要:

AbstractWhen the number of subjects in a two‐by‐two table is small or moderate, we may commonly use the exact conditional distribution with all marginals fixed to derive the conditional confidence limits on the underlying parameter. Under inverse sampling, in which we continue to sample subjects until we obtain exactly a pre‐determined number of subjects falling into a specific category, this paper notes that derivation of a confidence interval, which has the coverage probability equal to or larger than a nominal 1 – α confidence level, for relative risk and relative difference in cohort studies is straightforward. This paper further finds that, when the underlying disease is rare, we can similarly apply an inverse sampling to produce an approximate 1 – α conditional confidence limits on attributable risk in case‐control studies as well. When the number of subjects is small and the test statistic derived on the basis of large sample theory is not strictly adequate for use, this paper also presents an exact hypothesis testing procedure for the above parameters in the corresponding

ISSN:0277-6715    

DOI:10.1002/sim.4780141810

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

年代:1995

数据来源: WILEY

ISSN:0277-6715    

DOI:10.1002/sim.4780141801

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

年代:1995

数据来源: WILEY