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Choice of time scale and its effect on significance of predictors in longitudinal studies
Author(s) -
Pencina Michael J.,
Larson Martin G.,
D'Agostino Ralph B.
Publication year - 2006
Publication title -
statistics in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.996
H-Index - 183
eISSN - 1097-0258
pISSN - 0277-6715
DOI - 10.1002/sim.2699
Subject(s) - observational study , scale (ratio) , econometrics , regression analysis , statistics , time point , proportional hazards model , regression , event (particle physics) , framingham heart study , mathematics , computer science , medicine , framingham risk score , philosophy , physics , disease , pathology , quantum mechanics , aesthetics
Time‐to‐event regression is a frequent tool in biomedical research. In clinical trials this time is usually measured from the beginning of the study. The same approach is often adopted in the analysis of longitudinal observational studies. However, in recent years there has appeared literature making a case for the use of the date of birth as a starting point, and thus utilize age as the time‐to‐event. In this paper, we explore different types of age‐scale models and compare them with time‐on‐study models in terms of the estimated regression coefficients they produce. We consider six proportional hazards regression models that differ in the choice of time scale and in the method of adjusting for the years before the study. By considering the estimating equations of these models as well as numerical simulations we conclude that correct adjustment for the age at entry is crucial in reducing bias of the estimated coefficients. The unadjusted age‐scale model is inferior to any of the five other models considered, regardless of their choice of time scale. Additionally, if adjustment for age at entry is made, our analyses show very little to suggest that there exists any practically meaningful difference in the estimated regression coefficients depending on the choice of time scale. These findings are supported by four practical examples from the Framingham Heart Study. Copyright © 2006 John Wiley & Sons, Ltd.