When conducting research there are lots of factors to consider. Psychologists may want to study, for example, the effect of some new test on all college students, but this is obviously not possible. Instead, what they do is test on a sample or a smaller group of college students. In this example, everyone who could possibly be a participant in the study (meaning, all college students) is part of the population. College students would be the population the researcher wants to study and from which they select a sample.