摘要:

AbstractWhen measuring the joint effect of two factors it is advantageous to use a factorial design. If the application is suitable, efficiency may be further improved by using a crossover design. This paper presents a flexible method for amalgamating these two devices. Designs are constructed from smaller designs, known as bricks, generated cyclically from tabulated initial sequences. The bricks have known efficiencies for estimation of direct treatment main effects and interactions; the efficiencies can be simply combined to approximate the efficiencies of the whole design. This allows the user to build a design that is tailored to the particular objectives of the experiment. Three and four periods, and two factors with up to four levels are considered.

ISSN:0277-6715    

DOI:10.1002/sim.4780091002

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

年代:1990

数据来源: WILEY

摘要:

AbstractThis paper deals with model selection and the estimation of odds ratios from cross‐classified frequencies in the presence of extraneous factors. The odds ratio is estimated in different ways dependent on whether the extraneous factor is modelled as an effect modifier, a confounder, or neither. Routinely this choice is based on statistical tests of null hypotheses. By contrast, we propose selection of the model which is estimated to maximize the accuracy of the estimator of the odds ratio, on average. We demonstrate how a non‐parametric bootstrap method can be used to carry out the selection, and illustrate the methodology using an example on use of oral contraceptives and myocardial infarct

ISSN:0277-6715    

DOI:10.1002/sim.4780091003

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

年代:1990

数据来源: WILEY

摘要:

AbstractThe models of symmetry and quasi‐symmetry may have use in the analysis of categorical data in square tables where the row and column classifications are the same. Under the model of symmetry, the probability that an observation falls in, say, the (i, j) cell of a square table is the same as the probability that it falls in the (j, i) cell. The model of symmetry requires that the expected marginal total for any one row of the table be the same as the expected marginal total for the corresponding column. The model of quasi‐symmetry, which is a generalization of the model of symmetry, does not require this equality of expected row and column totals. In this paper, we illustrate the use of the models of symmetry and quasi‐symmetry in the analysis of categorical data that summarize pairs of sounds in infants' and toddlers' early utterances. In the past, the complexity of infants' early vocalizations has been difficult to measure because of a lack of methods to quantify prelinguistic phonetic variables. We use the models of symmetry and quasi‐symmetry as a first attempt to quantify the complexity of infants' early speech. We hope that the discovery of models appropriate for quantification of language development in normal children will have later use in identification of children at risk for communicative di

ISSN:0277-6715    

DOI:10.1002/sim.4780091004

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

年代:1990

数据来源: WILEY

摘要:

AbstractMultiplicative models for the association between blood pressure and coronary heart disease mortality in 5201 men aged 40–49 years at time of examination were compared. Piecewise exponential models fitted to summary tables formed by cross‐classification by three systolic and three diastolic blood pressure groups, two age groups and three time intervals gave results close to the Cox model for continuous data. The parametric integrated baseline hazard functions estimated from grouped data were close to Breslow's non‐parametric estimate in the Cox

ISSN:0277-6715    

DOI:10.1002/sim.4780091005

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

年代:1990

数据来源: WILEY

摘要:

AbstractThe maximum likelihood estimator (MLE) of the ratio of the hazard rates in two exponential distributions is biased. This bias can be important when sample sizes are small or the ratio of these two hazard rates is large. When there is either no censoring or type II censoring, we propose using the uniformly minimum variance unbiased estimator (UMVUE). We show that using the UMVUE instead of the MLE reduces the mean‐squared error (MSE). We have found that the UMVUE always has MSEs smaller than the MLE. We have also found that the UMVUE leads to important reductions in the MSE when the sample size used to calculate the hazard rate in the numerator of the hazard ratio is small (say ⩾ 20) regardless of the sample size in the denominator. In the presence of type I censoring, the proposed estimator and the MLE are both biased. On the basis of a Monte Carlo study, however, we obtain similar reductions in the MSE using the UMVUE, as for no censoring or type II censor

ISSN:0277-6715    

DOI:10.1002/sim.4780091006

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

年代:1990

数据来源: WILEY

摘要:

AbstractA primary outcome variable in longitudinal studies is often the rate of change of a continuous measurement over time. Examples include the one‐second forced expiratory volume (FEV1) in pulmonary studies or glomerular filtration rate (GFR) in renal studies. An individual patient's least‐squares estimate of slope obtained from a linear regression is an imprecise measure of the true slope for that patient, and correlations involving the estimated slopes will be biased due to this measurement error. This paper presents methods for estimating the true correlation between these imprecise slope estimates, or between slope estimates and other variables measured with error. In addition to providing a simple consistent estimator of the correlation, we show how the maximum likelihood estimate of the correlation coefficient and a 100(1 — α) per cent confidence interval can be obtained. An example estimating the correlation between GFR and inverse serum creatinine slopes in patients with chronic renal disease is

ISSN:0277-6715    

DOI:10.1002/sim.4780091007

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

年代:1990

数据来源: WILEY

摘要:

AbstractThe sensitivity of a prognostic scoring system will tend to be exaggerated if the scoring system is both derived and validated on the same data. This paper provides, by analogy to regression with error in an explanatory variable, an intuitive basis for the methodological results of Copas which seek to estimate the degree of such exaggeration. There was good agreement between Copas' results and those achieved in a series of cross‐validation exercises where logistic regression models predicting the risk of ischaemic heart disease were derived using data from the prospective British Regional Heart Study. When truly important variables were included, the exaggeration of the sensitivity increased as the number of cases of disease available decreased. It is concluded that Copas' method, which is easy to implement in practice, may be helpful in realistically anticipating the extent of such exaggeration, and that it can be usefully employed before pursuing a scoring system on newly collected dat

ISSN:0277-6715    

DOI:10.1002/sim.4780091008

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

年代:1990

数据来源: WILEY

摘要:

AbstractVariation in heart disease (HD) mortality rates across census tracts is greater than expected given binomial error and available explanatory variables. We extended an extra‐binomial variation model for rates standardized by the direct method. The overdispersion parameter accounted for 36 per cent of the observed variation in standardized rates. Ignoring overdispersion resulted in a change in an estimate of the effect of social class on HD mortality and substantial underestimation of the error of the estimates of such effects. Ecologic regression on the proportional mixture of social classes within tracts provided an appealing approach to the problem of estimating fixed effects with aggregated dat

ISSN:0277-6715    

DOI:10.1002/sim.4780091009

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

年代:1990

数据来源: WILEY

摘要:

AbstractThe simultaneous estimation of infection rate, cure rate and detectability of parasitic infections is considered. A new method for this estimation based on a simple statistical model assuming constant transition rates between parasite states is proposed. Repeated observations on the infection status of the same individuals is required for this method. A maximum likelihood approach is used for parameter estimation and the calculation of standard errors of the estimates. The method is illustrated by a longitudinal study of the presence ofGiardia lambliainfection in Kenyan children.

ISSN:0277-6715    

DOI:10.1002/sim.4780091010

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

年代:1990

数据来源: WILEY

ISSN:0277-6715    

DOI:10.1002/sim.4780091011

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

年代:1990

数据来源: WILEY