Biomedical subjects

David M Zucker

Publications and source records attributed to David M Zucker.

5 recordsLinked to original sources

PubMed2006

Inference for a family of survival models encompassing the proportional hazards and proportional odds models.

For survival data regression, the Cox proportional hazards model is the most popular model, but in certain situations the Cox model is inappropriate. Various authors have proposed the proportional odds model as an alternative. Yang and Prentice recently presented a number of easily implemented estimators for the proportional odds model. Here we show how to extend the methods of Yang and Prentice to a family of survival models that includes the proportional hazards model and proportional odds model as special cases. The model is defined in terms of a Box-Cox transformation of the survival function, indexed by a transformation parameter rho. This model has been discussed by other authors, and is related to the Harrington-Fleming G(rho) family of tests and to frailty models. We discuss inference for the case where rho is known and the case where rho must be estimated. We present a simulation study of a pseudo-likelihood estimator and a martingale residual estimator. We find that the methods perform reasonably. We apply our model to a real data set.

Chlamydia Infections↗

PubMed2005

Towards practical cognitive assessment for detection of early dementia: a 30-minute computerized battery discriminates as well as longer testing.

Early detection of cognitive decline may lead to more effective treatment. Clinical cognitive assessment is essential for early detection, but must be brief with easily interpretable results. The present study defines and evaluates a 30-minute cognitive battery consisting of a subset of tests that comprise a longer computerized battery recently validated in detecting mild cognitive impairment (MCI). Participants were from three tertiary care memory clinics and an assisted living facility (final group: N=161) with consensus diagnoses of cognitively healthy, MCI, or mild dementia. A comprehensive NeuroTrax battery evaluated memory, executive function, visual spatial perception, verbal function, information processing speed, and motor skills. Validity of a single summary measure ('MCI Score') designed for dementia detection and built exclusively from tests of memory, executive function, and visual spatial perception was evaluated with receiver operating characteristic (ROC) analysis. Discriminant validity (area under the curve: AUC) was at least as large for the 6-parameter MCI Score as for a 20-parameter score necessitating administration of the entire battery. Further, the MCI Score had a larger AUC with reduced variance relative to its constituent parameters. AUC for distinguishing dementia was 0.886; AUC for distinguishing cognitively healthy was 0.823. Finally, the MCI Score discriminated among all three diagnostic groups (ANOVA; F[2,150]=52.54, p<0.001). Hence a reduced NeuroTrax battery (30 minutes) with MCI Score is a useful clinical tool for summarizing cognitive data relevant to early dementia detection.

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PubMed2004

Inference for the proportional hazards model with misclassified discrete-valued covariates.

We consider the Cox proportional hazards model with discrete-valued covariates subject to misclassification. We present a simple estimator of the regression parameter vector for this model. The estimator is based on a weighted least squares analysis of weighted-averaged transformed Kaplan-Meier curves for the different possible configurations of the observed covariate vector. Optimal weighting of the transformed Kaplan-Meier curves is described. The method is designed for the case in which the misclassification rates are known or are estimated from an external validation study. A hybrid estimator for situations with an internal validation study is also described. When there is no misclassification, the regression coefficient vector is small in magnitude, and the censoring distribution does not depend on the covariates, our estimator has the same asymptotic covariance matrix as the Cox partial likelihood estimator. We present results of a finite-sample simulation study under Weibull survival in the setting of a single binary covariate with known misclassification rates. In this simulation study, our estimator performed as well as or, in a few cases, better than the full Weibull maximum likelihood estimator. We illustrate the method on data from a study of the relationship between trans-unsaturated dietary fat consumption and cardiovascular disease incidence.

Biometry↗

PubMed2002

Sample-size redetermination for repeated measures studies.

Clinical trialists recently have shown interest in two-stage procedures for updating the sample-size calculation at an interim point in a trial. Because many clinical trials involve repeated measures designs, it is desirable to have available practical two-stage procedures for such designs. Shih and Gould (1995, Statistics in Medicine 14, 2239-2248) discuss sample-size redetermination for repeated measures studies but under a highly simplified setup. We develop two-stage procedures under the general mixed linear model, allowing for dropouts and missed visits. We present a range of procedures and compare their Type I error and power by simulation. We find that, in general, the achieved power is brought considerably closer to the required level without inflating the Type I error rate. We also derive an inflation factor that ensures the power requirement is more closely met.

Analysis of Variance↗

PubMedDate not supplied

Computerized cognitive testing battery identifies mild cognitive impairment and mild dementia even in the presence of depressive symptoms.

Cognitive and depressive symptoms co-occur, complicating detection of mild cognitive impairment (MCI) and early dementia. In this study, discriminant validity of a novel computerized cognitive battery for MCI detection was evaluated after covariation for depressive symptom severity. In addition to the computerized battery, participants at two sites received the 30-item self-administered Geriatric Depression Scale (GDS; n=72); those at two other centers received the observer-administered Cornell Scale for Depression in Dementia (CSDD; n=88). In both cohorts, a Global Cognitive Score and memory, executive function, visual spatial, and verbal index scores discriminated among cognitively healthy, MCI, and mild dementia groups after covariation for GDS or CSDD, respectively (p < 0.05). Thus, the computerized battery for detection of mild impairment is robust to comorbid depressive symptoms, supporting its clinical utility in identifying neurodegenerative disease even in elderly with depression.

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