Project Summary: Sampling methods are divided into two broad categories: probability and nonprobability. The difference between probability and nonprobability sampling is the selection technique employed. Probability sampling includes any method of sampling that utilizes some form of random selection. In order to have a random selection method within probability sampling, you must set up some process or procedure that assures that the different units in your population have equal probabilities of being chosen. Nonprobability sampling, then, does not involve random selection, so researchers cannot know ahead of time the probability of an element being chosen. Though you might be tempted to conclude that nonprobability samples cannot be representative of a population, this is not necessarily the case.