In psychology, population describes the collective of individuals that a researcher aims to target. This may vary from all adolescents to as narrow as generalized anxiety disorder patients. Since it is often unreasonable to study a complete population, researchers tend to rely on representative samples.
For instance, a study concerning stress among college students may conclude that if a systematic sample of 200 students is drawn from a single institution, it would be appropriate to generalize the findings for the entire population of college students.
In applied psychology, population parameters are crucial due to the fact that an intervention that is successful for a particular group may not be effective for another group. This is not solely a methodological issue, but also an ethical issue where psychologists need to clear biases when defining or choosing study populations.