Non-random samples will usually not lead to accurate prediction. However, random samples help prediction tremendously. Large random samples help control against selection, a form of bias. Other types of samples also help make better predictions, they include matched samples and stratified random samples. Populations Populations are larger than samples.
Populations are the group about which predictions are made.
Inferential statistics are performed on samples and then the results inferred to populations because of the great practical difficulty in studying populations as a whole. However, automatic computer methods combined with universal Internet access may make population studies practical in the future. It is important to realize ahead of time what population you are studying because your inferences from sample to population may be different for different populations. For instance, a sample drawn from SAU students may lead to quite accurate predictions for the population of SAU students. It may even lead to fairly accurate predictions about the population of Arkansas undergraduate public college students, but not lead to accurate predictions about public college students on the East or West coasts of the United States.