ISSN:0277-6715    

DOI:10.1002/sim.4780140802

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

年代:1995

数据来源: WILEY

摘要:

AbstractI define three separate and distinct types of non‐random comorbidity: epidemiologic, clinical and familial. These might exist singly or in any combination for a pair of disorders. The focus is on their definition and the measurement and interpretation of the types of comorbidity of most common concern: epidemiologic comorbidity. I discuss certain sources of epidemiologic comorbidity such as shared risk factors, or diagnostic ‘fuzziness’, and I indicate the directions of research design and analyses to disclose such so

ISSN:0277-6715    

DOI:10.1002/sim.4780140803

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

年代:1995

数据来源: WILEY

摘要:

AbstractWe consider problems involving the comparison of two or more treatments where we have the opportunity to adjust for relevant covariates either conditionally in a regression model or implicitly in repeated measures data, for example, in crossover trials. It is seen that for data arising from non‐Normal distributions there is the possibility that models adjusting for covariates and those not adjusting for covariates will be inconsistent, that is, at most one of the models can be valid. Alternatively, even if conditional and unconditional models are valid, parameters in each model may have different interpretations. We note that this presents difficulties for the specification and interpretation of the analysis. It is also clear that model validation is critical. Specific attention is paid to survival data analysed by the Cox proportional hazards mode

ISSN:0277-6715    

DOI:10.1002/sim.4780140804

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

年代:1995

数据来源: WILEY

摘要:

AbstractReaction times for schizophrenic individuals in a simple visual tracking experiment can be substantially more variable than for non‐schizophrenic individuals. Current psychological theory suggests that at least some of this extra variability arises from an attentional lapse that delays some, but not all, of each schizophrenic's reaction times. Based on this theory, we pursue models in which measurements from non‐schizophrenics arise from a normal linear model with a separate mean for each individual, whereas measurements from schizophrenics arise from a mixture of (i) a component analogous to the distribution of response times for non‐schizophrenics and (ii) a mean‐shifted component. We fit four mixture models within this framework, where the distinctions between models arise from assumptions about the variance of the shifted observations and the exchangeability of schizophrenic individuals. Some of these models can be fit by maximum likelihood using the EM algorithm, and all can be fit using the ECM algorithm, where the covariance matrices associated with the parameters are calculated by the SEM and SECM algorithms, respectively. Bayesian model monitoring using posterior predictive checks is invoked to discard models that fail to reproduce certain observed features of the data and to stimulate the development of better

ISSN:0277-6715    

DOI:10.1002/sim.4780140805

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

年代:1995

数据来源: WILEY

摘要:

AbstractIn case‐control studies, cases are sampled separately from controls. In such studies the primary analysis concerns the estimation of the effect of covariables on being a case or a control. To explore causal pathways, further secondary analysis could concern the relationships among the covariables. In this paper the validity of such secondary analysis is addressed. In particular, the use of multiple logistic regression in case‐control studies where the dependent variable is not the case/control indicator is explored. It is shown that only under very restrictive conditions will sample regression coefficients correctly estimate their true value. In many situations, it may be valid to regress one covariable on others in the control group, but not in the case group or the combined sample. This principle is illustrated by a study of sexually transmitted disease in Ke

ISSN:0277-6715    

DOI:10.1002/sim.4780140806

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

年代:1995

数据来源: WILEY

摘要:

AbstractAntibody responses following vaccination usually are analysed by comparing geometric mean concentrations across levels of relevant covariates and by comparing the proportions of vaccinees responding. In the regression setting, the analyses are done on log‐transformed concentrations, estimating geometric mean responses conditional on a vector of covariates. More detailed analyses examining the relationship of covariates to different parts of the response distribution may be performed through the application of asymmetric least squares estimation of regression percentiles. We present a method for accounting for correlation in percentile regression analyses of longitudinal antibody response data. We illustrate the procedures with measles antibody response data from Haitian children who participated in a randomized trial of high titre vaccines. The strongest dose and strain effects were seen in the low end of the antibody concentration distribution

ISSN:0277-6715    

DOI:10.1002/sim.4780140807

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

年代:1995

数据来源: WILEY

摘要:

AbstractIt is well known that when uncorrelated measurement error affects both variables in linear regression, there is attenuation of the correlation coefficient and regression slope. The effect of correlated measurement error, however, has received little attention. In medical method comparison studies, such error correlation results from the presence of other, unknown explanatory variables that affect the results of the new test method and the reference test method to which it is being compared. The contribution of correlated measurement error to the observed correlation coefficient can be accounted for by the expression ρt1t2= ρ1ρ2+ ρE1E2(1–ρ 12)1/2(1–ρ 22)1/2where ρt1t2is the observed correlation between tests 1 and 2, ρ1and ρ2are the correlation with true values for tests 1 and 2, respectively, and ρE1E2is the correlation between the test errors. The first term describes the attenuation due to uncorrelated error, the second term describes the effect of correlated error. A positive correlation between the measurement errors reduces the attenuation of observed correlation and slope, but, when the reference method is excellent, the effect is very small. For poorer reference tests whose correlations with true values are less than 0·9, however, error correlation may result in a slope and correlation coefficient that differ importantly from the values obtained with either uncorrelated error or with no reference test error. Negatively correlated measurement errors magnify the attenuation of slope and correlation. One might suspect the presence of correlated error when the observed regression slope is close to or exceeds 1 and the reference test is known to have suboptimal reliability. This paper provides several clinical examples of potentially correlated

ISSN:0277-6715    

DOI:10.1002/sim.4780140808

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

年代:1995

数据来源: WILEY

摘要:

AbstractWe present a new method of detection and inference for spatial clusters of a disease. To avoidad hocprocedures to test for clustering, we have a clearly defined alternative hypothesis and our test statistic is based on the likelihood ratio. The proposed test can detect clusters of any size, located anywhere in the study region. It is not restricted to clusters that conform to predefined administrative or political borders. The test can be used for spatially aggregated data as well as when exact geographic co‐ordinates are known for each individual. We illustrate the method on a data set describing the occurrence of leukaemia in Upstate New Yor

ISSN:0277-6715    

DOI:10.1002/sim.4780140809

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

年代:1995

数据来源: WILEY

摘要:

AbstractThe logarithmic (log) transformation is a simple yet controversial step in the analysis of positive continuous data measured on an interval scale. Situations where a log transformation is indicated will be reviewed. This paper contends that the log transformation should not be classed with other transformations as it has particular advantages. Problems with using the data themselves to decide whether or not to transform will be discussed. It is recommended that log transformed analyses should frequently be preferred to untransformed analyses and that careful consideration should be given to use of a log transformation at the protocol design stage.

ISSN:0277-6715    

DOI:10.1002/sim.4780140810

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

年代:1995

数据来源: WILEY

摘要:

AbstractA long term increase in incidence of and mortality due to malignant melanoma has been observed in all well documented white populations. The major identified cause of melanoma is sun exposure. One would expect predictions of future atmospheric ozone depletion to lead to an increase in ultraviolet radiation (UVR) and in the effects of sun exposure. We consider age‐period data for Canadian malignant malanoma mortality. We fit a multiplicative exponential/logistic (MEL) model to the data and extrapolate to AD 2010 hence yielding point estimates of future rates. We obtain total mortality forecasts by multiplying rates by population estimates. We present standard errors for forecasts. We forecast that melanoma will be a much larger burden on the health care system in the early years of the next century than it is at present. We obtain an age‐cohort model by a simple transformation of the age‐period model. Also, we obtain unconditional probabilities of death due to melanoma both for age‐period and age‐cohort models. We discuss the assumptions underlying the MEL model that suggest possible relationships between UVR and

ISSN:0277-6715    

DOI:10.1002/sim.4780140811

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

年代:1995

数据来源: WILEY