摘要:

AbstractWe compare balanced randomization with four adaptive treatment allocation procedures in a clinical trial involving two treatments. The objective is to treat as many patients in and out of the trial as effectively as possible. Randomization is a satisfactory solution to the decision problem when the disease in question is at least moderately common. Adaptive procedures are more difficult to use, but might play a role in clinical research when a substantial proportion of all patients with the disease are included in the trial.

ISSN:0277-6715    

DOI:10.1002/sim.4780140302

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

年代:1995

数据来源: WILEY

摘要:

AbstractClinical research often involves continuous outcome measures, such as blood cholesterol, that are amenable to statistical techniques of analysis based on the mean, such as thet‐test or multiple linear regression. Clinical interest, however, frequently focuses on the proportion of subjects who fall below or above a clinically relevant cut‐off value, as a measure of the risk of disease. The customary approach to analyse such data is to dichotomize the continuous outcome measure and use statistical techniques based on binary data and the binomial distribution. In this paper, we use a parametric approach and the framework of generalized linear models to fit various regression models, including the logistic, on the basis of the original continuous outcome. We consider the Gaussian and the three‐parameter log‐normal distributions for the continuous outcome, assessing both precision and bias under various conditions. In simulation analyses, we find that we are unable to fit some of the samples with the ‘dichotomous’ approach, but we can with the ‘continuous’ approach, and that the latter yields estimates between 25 and 85 per cent more efficient than the former. We illustrate the method, programmed using GLIM macros, with data from clinical studies of the risk of hypoxaemia during open thoracic surgery and the risk of nocturnal hypoglycaemia among di

ISSN:0277-6715    

DOI:10.1002/sim.4780140303

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

年代:1995

数据来源: WILEY

摘要:

AbstractThe visual acuities of 1140 children between the ages of 2 and 9 years were measured using the Sonksen—Silver Acuity System (SSAS) from a distance of 3 metres. Ordered logistic regression was used to construct smoothly changing age‐related reference ranges for the proportions of children at different ages expected to have acuities in each of three ordinal categories. Exponential functions of age fitted the data better than polynomials. The resulting reference ranges can be used to derive the ages at which a given percentage of children achieve each of the acuity cut‐offs. However, since it is unlikely that all children will be available for testing at the exact appropriate age, two possible alternatives are introduced and discussed. First, a new ordinal category that would result in a chosen outcome at a pre‐selected age is calculated. Secondly, because it may be more feasible to use the current SSAS system whilst varying the distance of measurement, age‐related standards showing the distances required to give constant pass‐rates with the usual letter‐sizes are a

ISSN:0277-6715    

DOI:10.1002/sim.4780140304

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

年代:1995

数据来源: WILEY

摘要:

AbstractThe mathematical multistage model of carcinogenesis proposed by Breslow and Day indicates that if hormonal risk factors for breast cancer are late‐stage carcinogens then the use of an additive model (excess absolute risk) should be preferrable to the present standard use of the multiplicative model (excess relative risk). To test different models, information from a large prospective study of 431,604 women aged 45–74 years in 1970 with follow‐up to 1985 on reproductive factors and post‐menopausal breast cancer mortality was used to compute goodness‐of‐fit statistics. The relative risk function, ranging from multiplicative to additive, was explored by changing the exponent in a power transformation. The analysis found evidence of better fit by the additive compared with the multiplicative models, consistent with the proposed mathema

ISSN:0277-6715    

DOI:10.1002/sim.4780140305

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

年代:1995

数据来源: WILEY

摘要:

AbstractWe propose a non‐parametric method to calculate a confidence interval for the difference or ratio of two median failure times for paired observations with censoring. The new method is simple to calculate, does not involve non‐parametric density estimates, and is valid asymptotically even when the two underlying distribution functions differ in shape. The method also allows missing observations. We report numerical studies to examine the performance of the new method for practical sample si

ISSN:0277-6715    

DOI:10.1002/sim.4780140306

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

年代:1995

数据来源: WILEY

摘要:

AbstractMany longitudinal studies and clinical trials are designed to compare rates of change over time in one or more outcome variables in several groups. Most such studies have incomplete data because some patients drop out before completing the study. The missing data may induce bias and inefficiency in naive estimates of important parameters. This paper uses Monte Carlo methods to compare the bias and efficiency of several two‐stage estimators of the effect of treatment on the mean rate of change when the missing data arise from one of four processes. We also study the validity of confidence intervals and the power of hypothesis tests based on these estimates and their standard errors. In general, the weighted least squares estimator does relatively well, as does an analysis of covariance type estimator proposed by Wuet al.The best estimates of variance components are based on complete cases or maximum likelihoo

ISSN:0277-6715    

DOI:10.1002/sim.4780140307

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

年代:1995

数据来源: WILEY

摘要:

AbstractCorrection for chance‐expected agreement has become an accepted technique in the analysis of observer agreement data and may be particularly useful when the level of agreement achieved in different populations is compared. However, formal methods for making comparisons of chance‐corrected agreement or, more generally, for studying the effects of covariates on chance‐corrected agreement have not received much attention. For nominal scale agreement data we show how Tanner and Young's model for observer agreement can be applied to this problem. The models discussed can be fitted using existing software and certain model parameters have interpretations in terms of positive and negative agreement odds ratios. The proposed methodology facilitates investigation of issues such as confounding of covariate effects and interaction between covariates in their effect on chance‐corrected agreement. The methods outlined therefore allow observer agreement data to be analyzed in a manner strongly analogous to the logistic modelling of the association between disease and suspected risk factors. The methods are illustrated using data on the comparability of primary and proxy respondent reports of the primary respondents participation in physically vigorous leisure time a

ISSN:0277-6715    

DOI:10.1002/sim.4780140308

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

年代:1995

数据来源: WILEY

摘要:

AbstractThere is general recognition that some glands secrete hormones primarily as a series of pulses. One can generally classify the proposed methods of pulse identification and characterization as either (i) criterion‐based, that is, they use a criterion such as a test statistic to identify a rise and/or fall in hormone level within a moving window, or (ii) model‐based, that is, they specify a statistical model for the time‐varying portion of the signal and estimate its parameters.Using simulated data, we compare and contrast seven criterion‐based methods and three model‐based methods. The model‐based methods perform better in estimating the parameters of interest; they are most effective with the sampling rate chosen so that there are 3–5 samples taken during the half‐life of the hormone. At higher sampling rates the methods may overidentify pulses (false positives) and at lower sampling rates they may miss pulses (false negatives), both of which lead to biased estimates for the parameter

ISSN:0277-6715    

DOI:10.1002/sim.4780140309

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.4780140310

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.4780140301

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

年代:1995

数据来源: WILEY