Donald Hedeker's chief expertise is in the development and use of advanced statistical methods for clustered and longitudinal data, with particular emphasis on mixed-effects models. He is the primary author of several freeware computer programs for mixed-effects analysis. With Robert Gibbons, Don is the author of the text “Longitudinal Data Analysis,” published by Wiley in 2006. More recently, he has developed methods and software for analysis of intensive longitudinal data, which are data with many measurements over time, often collected using mobile devices and/or the internet. Such data are increasingly obtained by researchers in many research areas, for example in the areas of mobile health (mHealth) and ecological momentary assessment (EMA) studies. He is an associate editor for Statistics in Medicine.