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R Crouchley

Publications and source records attributed to R Crouchley.

11 recordsLinked to original sources
PubMed1998

Interpreting the beta geometric in comparative fecundability studies.

The beta distribution has often been used to describe variation in fecundability between couples when modelling the number of menstrual cycles required for a couple to achieve pregnancy. Groups are often compared for a condition or factor that could affect conception rates. However, when heterogeneity is present in each group, comparing group-specific parameters is not very informative on what any differences might imply for the individual couples. We present a quantile ratio that provides an individual level interpretation of any between-group differences in the beta distribution. Our results are illustrated on the two examples of Ridout and Morgan (1991, Biometrics 47, 1423-1433). The quantile ratio suggests a simplification in one of the examples.

Biometry↗
PubMed1995

A comparison of frailty models for multivariate survival data.

This paper reviews some of the main approaches to the analysis of multivariate censored survival data. Such data typically have correlated failure times. The correlation can be a consequence of the observational design, for example with clustered sampling and matching, or it can be a focus of interest as in genetic studies, longitudinal studies of recurrent events and other studies involving multiple measurements. We assume that the correlation between the failure or survival times can be accounted for by fixed or random frailty effects. We then compare the performance of conditional and mixture likelihood approaches to estimating models with these frailty effects in censored bivariate survival data. We find that the mixture methods are surprisingly robust to misspecification of the frailty distribution. The paper also contains an illustrative example on the times to onset of chest pain brought on by three endurance exercise tests during a drug treatment trial of heart patients.

Angina Pectoris↗
PubMed1994

Survival models for developmental genetic data: age of onset of puberty and antisocial behavior in twins.

The use of survival analysis for developmental genetic data is discussed. The main requirements for models based on the decomposition of frailty distributions into shared and unshared components are outlined for the simple case of twins. Extending the earlier work of Clayton, Oakes, and Hougaard, among others, three forms of hazard model are presented, all of which can be applied to pedigree data with flexible baseline hazards without the use of numerical integration. The first two models use an additive decomposition of frailty, with either gamma or positive stable law distributed (PSL) components. The third model previously described by Hougaard involves a multiplicative PSL decomposition. The models are applied to data on the onset of puberty in male twins and illustrate the importance of correct specification of the baseline hazard for correct inference about genetic effects. The difficulty of assessing model specification using information only on the margins is also noted. Overall, the new model with additive PSL components appeared to fit these data best. A second application illustrates the use of a time-varying covariate in examining the impact of puberty on the onset of conduct disorder symptomotology.

Adolescent↗
PubMed1994

A simple method for censored age-of-onset data subject to recall bias: mothers' reports of age of puberty in male twins.

Genetic analysis of variation in age of onset of development milestones or psychopathological behaviors has been little researched, owing largely to the computational difficulty of dealing with "censored" observations. Censored observations arise when the only information on individuals is that they have reached a particular age but without onset having occurred. This paper shows how models can be simply fitted to such data using programs that can perform genetic analysis of categorical data by maximum likelihood. The method is illustrated using the program Mx with data on maternal report of the onset of puberty in twin sons from the Virginia Twin Study of Adolescent Behavioral Development. Frequently, data on age of onset is collected by retrospective recall. This can pose a variety of measurement problems. Suggestions are made for models that account for some of these problems or are robust to their presence. Substantial evidence for "telescoping" of onset dates is found for the puberty data. If left unaccounted for, these effects can artifactually inflate estimates of common environment effects.

Adolescent↗
PubMed1994

Generalizations and applications of frailty models for survival and event data.

A variety of survival models with both discrete and continuously distributed frailty is considered within a framework that involves the specification of three sub-models. An intensity sub-model specifies how the intensity is related to values of covariates and frailty; a measurement sub-model specifies how fallible measures of frailty are related to it; and an exposure sub-model specifies how frailty is distributed within the population. The models include those in which frailty is due to omitted covariates and those where it represents a covariate that has been measured subject to error. Multivariate frailty is also considered, with particular emphasis on models suitable for application to genetically related individuals, notably twins. A numerical example illustrates the use of a model with multivariate frailty for data on repeated exercise times.

Angina Pectoris↗
PubMed1993

A specification test for univariate and multivariate proportional hazards models.

This paper applies White's (1982, Econometrica 50, 1-25) information matrix (IM) test for correct model specification to proportional hazards models of univariate and multivariate censored survival data. Several alternative estimators of the test statistic are presented and their size performance examined. White also suggested an estimator of the parameter covariance matrix that was robust to certain forms of model misspecification. This has been subsequently proposed by others (e.g., Royall, 1986, International Statistical Review 54, 221-226) and applied by Huster, Brookmeyer, and Self (1989, Biometrics 45, 145-156) as part of an independence working model (IWM) approach to multivariate censored survival data. We illustrate how the IM test can be used for both univariate data and as part of the IWM approach to multivariate data.

Biometry↗
PubMed1984

Calibrating longitudinal models of residential mobility and migration: an assessment of a non-parametric marginal likelihood approach.

"This paper discusses the problems of controlling for omitted variables in estimating the structural parameters of longitudinal models and focuses upon an assessment of a non-parametric marginal maximum likelihood approach suggested by the results of Laird.... The approach is shown to be statistically valid for a plausible discrete-time model of the incidence of residential or migration moves, at least for data in which no household moves in every time period. Empirical evaluation with two large [U.S.] datasets on residential mobility indicates that the approach is also computationally feasible and provides a promising alternative to more conventional methods for controlling for omitted variables."

Americas↗
PubMed1984

Methods for the identification of Lexian, Poisson and Markovian variations in the secondary sex ratio.

One of the problems arising in the identification of the type of process generating the secondary sex ratio in human and other populations is that of 'spurious contagion'. A model is presented which allows the simultaneous estimation of the effects of (i) variation in the biological predisposition of couples to have children of a particular sex, (ii) variation in this predisposition with birth order, (iii) a Markov association between the sexes of adjacent sibs as the result of in utero interaction, and (iv) the socio-psychological predisposition of couples to limit families once they have obtain a desirable composition. Restricted forms of the model allow one to test for the presence of each of these effects whilst controlling for the others. The model is applied to data from the United States and Finland.

Birth Order↗
PubMed1982

Heterogeneity, nonstationarity, and duration-of-stay effects in migration.

"This paper briefly outlines the theoretical problem of the identification of duration-of-stay effects in migration. The empirical studies are critically reviewed with particular emphasis upon their treatment of heterogeneity and nonstationarity. New statistical tests developed by the authors are applied to Wisconsin migration data. These tests suggest that although heterogeneity and nonstationarity are strongly present within the data, duration-of-stay effects appear weak."

Americas↗
PubMed1982

Modelling the evolution of heterogeneity in residential mobility.

Migration probabilities are known to vary over the population (heterogeneity) and over time (nonstationarity). It is shown that if the heterogeneity is represented by an SB mixing distribution and the nonstationarity by a set of arbitrary logistic scaling functions, then not only may all sources of heterogeneity, including tastes, be modelled, but heterogeneity at different points in time may be readily compared. The model is calibrated using data previously published by Clark et al. (1977, 1979) for a sample of 1,176 older renters and is found to fit well. The changing heterogeneity over the ten-year period of observation is represented graphically.

Demography↗