The subset of eHRAF World Cultures known as the Probability Sample Files (PSF) is a special kind of random sample called a stratified random sample. The world was divided into 60 culture areas (strata) and one case from each area was randomly chosen from a list of mostly preindustrial societies that met certain quality control criteria (such as whether one of the ethnographers stayed for more than a year). All 60 of the PSF cases are included in the paper, microfiche, and online versions of the HRAF Collection of Ethnography. For the rules used in selecting the sample cases, see Naroll, Raoul. 1967. The proposed HRAF Probability Sample. Behavior Science Notes 2: 70-80.