摘要:

AbstractEstimates of transmission probabilities for sexually transmitted diseases historically come from studies of uninfected individuals exposed to those with a high disease prevalence (for example, prostitutes). However, changes in sexual behaviour, much of which relates to concerns about AIDS, has made identification of populations suitable for such studies extremely difficult. This paper presents a method for estimating these probabilities that utilizes a deterministic model and routinely collected data available in many clinics. Variance estimates for the estimators are also derived. Data for chlamydial infection and sensitivity analyses for the input parameters and assumptions illustrate the method.

ISSN:0277-6715    

DOI:10.1002/sim.4780110502

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

年代:1992

数据来源: WILEY

摘要:

AbstractThis paper presents a model for analysing the incubation period of highly infectious diseases in populations where almost all susceptibles become infected during an epidemic. The model leads to a simple method for estimating the variance of the duration of the incubation period without any distributional assumptions. Further, the influence of covariates on the duration of the incubation period can be analysed. Data from the epidemic of measles in Greenland in 1951 are analysed and it is found that intersymptom times are correlated within households, suggesting that secondary cases are infected almost simultaneously. This result is inconsistent with a variation in the times of infection of secondary cases within a household which is often assumed when analysing data on measles. Prophylactic treatment did not prevent infection in the epidemic in Greenland, but it is found that the incubation period tended to be shorter for persons not receiving prophylactic treatment.

ISSN:0277-6715    

DOI:10.1002/sim.4780110503

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

年代:1992

数据来源: WILEY

摘要:

AbstractThe purpose of this study was to estimate seroconversion time using different parametric methods and to assess their influence on the estimation of the incubation time between HIV infection and onset of AIDS. Study subjects were 712 HIV‐positive haemophiliacs enrolled in the Italian National Registry of patients with congenital coagulation disorders. Seroconversion time was estimated using the mid‐point of each seroconversion interval (MID), the median of each interval under an estimated uniform distribution with cutpoints at December 1981 and December 1985 (MUU), the median of each interval under an estimated Weibull distribution (MUW), and the median of three random values drawn from each interval under the Weibull distribution (RUW). Kaplan‐Meier survival analysis showed that the cumulative incidence of AIDS over a 7‐year period was 11·6 per cent (SE 1·3 per cent) when using the MID estimate of seroconversion time, 10·8 per cent (1·2 per cent) with the MUU estimate, and 13·4 per cent (1·3 per cent) and 12·3 per cent (1·3 per cent) when using MUW and RUW estimates, respectively. This study demonstrates that the estimate of seroconversion time does not seem to be a major factor affecting estimates of AIDS incidence since the different techniques for estimating HIV seroconversion time yielded very

ISSN:0277-6715    

DOI:10.1002/sim.4780110504

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

年代:1992

数据来源: WILEY

摘要:

AbstractThe usual form of the Mantel–Haenszel test statistic assumes independent observations. This is inappropriate for data from a stratified multistage survey. Two alternative adjustments to the test statistic are developed to deal with this: (a) a modification of the effective sample size for each row of each table, using the design effects, extending a method proposed by Donald and Donner for familial aggregation studies; (b) a Taylor series approximation to the variance of the square root of the numerator of the Mantel‐Haenszel statistic. Both methods are evaluated by application to both simulated and real data. The two methods perform equally well, and offer a considerable improvement over the unadjusted test statistic when observations from the same cluster are highly correlated. A simplified adjustment is also considered, as is the need for correction to the variance of the odds ratio estima

ISSN:0277-6715    

DOI:10.1002/sim.4780110505

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

年代:1992

数据来源: WILEY

摘要:

AbstractMany matched case‐control studies present tables of pooled data ignoring matching. The value of such tables is questioned. If tables of data are included they should certainly not lead to misleading results. Tables that yield consistent estimates in matched studies are considere

ISSN:0277-6715    

DOI:10.1002/sim.4780110506

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

年代:1992

数据来源: WILEY

摘要:

AbstractWe apply parametric empirical Bayes inference of Morris7to the estimation of rate of change from incomplete longitudinal studies where the right censoring process is considered informative, that is, the length of time the subjects participate in the study is associated with level of the study variable. Ignoring such an association can result in a biased estimate of rate of change. The proposed method provides estimates of rate of change for individual subjects as well as for the entire group, adjusted for informative right censoring. The method is considered more robust than those based on a specific parametric model for the censoring distribution. Under non‐informative right censoring these estimators of slopes are equivalent to the Bayes estimators derived by Fearn.11We illustrate the method with an example involving renal transplant data. We evaluate the method's performance through a simulation stud

ISSN:0277-6715    

DOI:10.1002/sim.4780110507

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

年代:1992

数据来源: WILEY

摘要:

AbstractIn this paper we extend Bloch's discussion on the usefulness and the limitations in the application of repeated measurements per subject in study designs. We derive general sample size formulae for any finite number of comparison groups to calculate the required number of subjects with repeated measurements, that do not have to be conditionally independent. For fixed total cost, we discuss the optimal sample allocation for repeated measurements needed to maximize the power and the underestimation when using Bloch's sample size formula if in the hypothesis testing procedure the variance parameters are unknown. We have also included a quantitative investigation of the effectiveness of taking repeated measurements per subject to reduced the required number of subjects for a given power at a given α‐lev

ISSN:0277-6715    

DOI:10.1002/sim.4780110508

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

年代:1992

数据来源: WILEY

摘要:

AbstractOne method for analyzing contingency tables with missing observations is to model the missing‐data mechanism using log‐linear models. Previous methods for obtaining estimates (of missing counts and parameters) have required an iterative algorithm. In many cases, however, one can obtain estimates by use of a simple algebraic formula. We illustrate the method with data on smoking and birth wei

ISSN:0277-6715    

DOI:10.1002/sim.4780110509

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

年代:1992

数据来源: WILEY

摘要:

AbstractConditional probability procedures offer a flexible means of performing sequential analysis of clinical trials. Since these procedures are not based on repeated significance tests, the number and schedule of the interim analyses is less important than with group sequential procedures. Their main disadvantage is that the magnitude of their effect on the significance level is difficult to assess. This paper describes a conditional probability procedure which attempts to maintain the overall significance level by balancing the probabilities of false early rejection and false early acceptance. Monte Carlo sampling results suggest that this procedure can achieve a large reduction in expected sample size without greatly affecting either the significance level or power of the trial.

ISSN:0277-6715    

DOI:10.1002/sim.4780110510

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

年代:1992

数据来源: WILEY

摘要:

AbstractWe consider the decision‐theoretic evaluation of early stopping rules for clinical trials. We describe a hypothetical phase III, two‐arm trial with interim analysis, modelling it as a decision problem. We present methods for constructing a group‐sequential test that is optimal for a given utility and prior distribution, and for determining a utility and prior under which a given test is optimal. Using our methods, we construct optimal tests for utilities and priors representing a variety of perspectives: a view that seeks to maximize the long‐run response rate, a view that seeks to maximize the chance of a correct decision, and a personal view elicited from a medical oncologist. Characteristics of the optimal rules are sensitive to the input utilities and priors, and their critical values and sampling properties can differ markedly from those of group‐sequential tests. We also compute utility functions and priors under which some standard group‐sequential stopping rules are optimal. Our results suggest that the group‐sequential tests, considered as decision procedures, imply a symmetry in the decision problem that is inconsistent with the response‐rate perspective. We conclude that decision theory is a useful device for illuminating the sometimes conflicting goals of clinical research, and that the use of decision theory in designing clinical trials could lead to marked changes in statistical criteria for ea

ISSN:0277-6715    

DOI:10.1002/sim.4780110511

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

年代:1992

数据来源: WILEY