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A sample is a subset chosen from a population for investigation. A "random sample" is one chosen by a method involving an unpredictable component, in the sense that the selection of any element of the population was independent of the selection of any other element. A "simple random sample" is one in which every item/object in the larger population of interest had an equal opportunity/chance/probability of being selected. A '"probability sample" is one in which each item has a known probability of being in the sample.