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 in the world (which would be considered the "population"), but this is obviously not possible. Instead, what they do is test a smaller group of college students, known as a sample. In this example, everyone who could possibly be a participant in the study (meaning, all college students) is part of the population. Thus, a sample is a relatively small number of participants drawn from an entire population.
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