ABSTRACT
Researchers often collect multiple observations from many individuals. For example, in research examining the relationship between stress and mood, a research participant may complete measures of both these variables every day for several weeks, and so daily measures are grouped within participants. In relationship research, a respondent may report on characteristics of his or her interactions with a number of different friends. In developmental research, individuals may be measured at many different times as they develop. In cognition research, reaction times may be observed for multiple stimuli.
