ABSTRACT
Repeated measures or longitudinal studies are broadly defined as studies for which a response variable is observed on at least two occasions for each experimental unit. These studies generate serial measurements indexed by some naturally scaled characteristic such as age of experimental unit, length of time in study, calendar date, trial number, student's school grade, individual's height as a measure of physical growth, or cumulative exposure of a person to a risk factor. The experimental units may be people, white rats, classrooms, school districts, or any other entity appropriate to the question that a researcher wishes to study. To simplify language, we will use the term "subject" to reference the experimental units under study and the term "time" to reference the longitudinal indexing characteristic.
