Biomedical subjects

Timothy R Johnson

Publications and source records attributed to Timothy R Johnson.

3 recordsLinked to original sources

PubMed2006

Generalized linear models with ordinally-observed covariates.

An ordinally-observed variable is a variable that is only partially observed through an ordinal surrogate. Although statistical models for ordinally-observed response variables are well known, relatively little attention has been given to the problem of ordinally-observed regressors. In this paper I show that if surrogates to ordinally-observed covariates are used as regressors in a generalized linear model then the resulting measurement error in the covariates can compromise the consistency of point estimators and standard errors for the effects of fully-observed regressors. To properly account for this measurement error when making inferences concerning the fully-observed regressors, I propose a general modelling framework for generalized linear models with ordinally-observed covariates. I discuss issues of model specification, identification, and estimation, and illustrate these with examples.

Analysis of Variance↗

PubMed2004

A generalized estimating equations approach to mixed-effects ordinal probit models.

Clustered ordinal responses, which are commonplace in behavioural and educational research, are often analysed using mixed-effects ordinal probit models. Likelihood-based inference for these models can be computationally burdensome, and may compromise the consistency of estimators if the model is misspecified. We propose an alternative inferential approach based on generalized estimating equations. We show that systems of estimating equations can be specified for mixed-effects ordinal probit models that avoid the potentially heavy computational demands of maximum likelihood estimation, and can also provide inferences that are robust with respect to some forms of model misspecification--particularly serial effects in longitudinal data.

Behavior↗

PubMedDate not supplied

Comparing health status with healthy habits in elderly assisted-living residents.

There is a growing number of older adults in America. Assisted-living facilities have become a more prevalent residence for this increasing population. In the study described in this article, a secondary data analysis was completed using residents of several LifeTrust America, Inc.,'s assisted-living facilities across the Southeastern United States. There were 1,079 residents who completed the Healthy Generation Survey. Residents were asked whether they currently, previously, or had never had certain health conditions. They also responded to various questions about their current participation in healthy habits, such as using cigarette or tobacco products, moderate alcohol consumption, eating breakfast, eating healthy snacks, getting 7-8 hours of sleep each night, drinking the daily recommended amount of water, and participating in regular physical exercise. The research question asked if there was a relationship between current healthy habits and physical health. The study found that individuals with more current physical health conditions also reported greater participation in healthy habits. This could lead to the conclusion that after individuals acquired certain physical health conditions, they began to participate in more healthy habits. Also of significance was the finding that participation in physical exercise and recreational activity related to greater participation in all other healthy habits.

Age Distribution↗