Within- and Between-Group Effects
An important concept to understand with multilevel data is the distinction between between-group and within-group effects. For instance, larger animal species (elephants) tend to live longer than smaller species (ducks); this is the between-group effect of size on life expectancy. Within a species, however, larger individuals (big ducks) tend to live less long than smaller individuals (small ducks); this is the within-group effect of size on life expectancy. Recently, Shankar Vedantan described another example on NPR: people report being happier making $50 for a project if their co-workers are making $40 versus making $60 for the same project if their co-workers are making $70. See the full story.