APA Dictionary of Psychology

the selection of study units (e.g., participants, homes, schools) from a larger group (population) in an unbiased way, such that the sample obtained accurately reflects the total population. For example, a researcher conducting a study of university admissions would need to ensure they used a representative random sample of schools—in other words, each school would have an equal probability of being chosen for inclusion, and the group as a whole would provide an appropriate mix of different school characteristics (e.g., private or public, student body size, cost, proportion of students admitted, geographic location).