Method in research for extracting samples from a population where each individual has an equal opportunity for selection, and the selection of a subject has no influence on the selection of other subjects. Random selection is used to avoid the systematic error that can occur from other strategies. Random selection is not the same as random assignment, which addresses to which group the selected subjects will be placed. In practice, true random selection from the larger population is difficult to accomplish in PDD research. Either subjects are drawn from subgroups, such as military recruits or college students, or they self-select in proportions that may not well represent the larger population, such as through newspaper recruitments.