Donald Hedeker, Ph.D., is professor of biostatistics in the Department of Public Health Sciences of the University of Chicago. He received his Ph.D. in Quantitative Psychology from the University of Chicago in 1989. His 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. In 2000, Dr. Hedeker was named a Fellow of the American Statistical Association. He is an associate editor for Statistics in Medicine and Journal of Statistical Software. He has been the principal investigator (PI), co-PI, or co-investigator on many research grants funded by the National Institutes of Health.