Design and Analysis of Longitudinal Studies.

PSYCHIATRIC ANNALS 38:12 | DECEMBER 2008 In 1993, several of the authors of this issue of Psychiatric Annals wrote an article titled “Some Conceptual and Statistical Issues in the Analysis of Longitudinal Psychiatric Data.” The article provided a timely discussion and illustration of newly developed mixed-effects regression models for the analysis of longitudinal psychiatric data. The article has now been cited more than 500 times. Not only has the article been used to inform the psychiatric research community, but it has been widely cited throughout the medical literature. Well, we’re back, and in this issue, we focus on topics that have arisen in the analysis of longitudinal data over the past 15 years that we as statisticians believe are important for the progress of medical research in general, and psychiatric research in particular. Specifi cally, we focus on sample size determination for clustered and/or longitudinal studies; the role of the intent-to-treat principle in longitudinal studies and various alternatives; conceptual and experimental design issues related to missing data in longitudinal studies; advances in the analysis of longitudinal data that insulate inferences from the effects of missing data; and balancing treatment and comparison groups in longitudinal studies. Each of these areas has been the focus of considerable statistical research, and advances made in these areas