In simple random samples (SRSs), every unit of a population has an equal probability of being selected for the sample. On the left, we see 12 potential units in the population (Each given a unique identifier from one to twelve). 4 of these units (numbers 2, 5, 8, and 10) were selected by the sample at random. 

SRSs are infrequent in the social sciences, but they are important in principle and essential for understanding the difference between a sample and a population.