group effect
a research finding specific to the group of individuals to which a participant belongs. A group effect could appear in an assigned subset, such as a treatment or intervention, or in a naturally occurring subset, such as age level or classroom. For example, a researcher might be interested in a group effect of a specific reading intervention, or in a group effect of book reading for students in the current decade who may be reading less than previous sets of students owing to more common use of the computer and television.