A probability sample is a sample selected by a method based on the theory of probability (random process), that is, by a method involving knowledge of the likelihood of any unit being selected Any method of selection of a sample based on the theory of probability; at any stage of the operation of selection the probability of any set of units being selected must be known. It is the only general method known which can provide a measure of precision of the estimate. Sometimes the term random sampling is used in the sense of probability sampling.