The idea is that multiple categories and limited conclusions are superior to indefensible generalizations on 'make-do' probability samples. It is better to sample LA, Bakersfield, and Sacramento cops and generalize results across these cities than to do a flawed probability sample of all cops in California and try to generalize to. This is the only statistically justifiable basis for external validity. Survey methods are uniquely well suited to this sampling technique. Unit of analysis can be individual, group, organization, or artifact.
Every unit has the same non-zero chance of being included in the study.
Non-Probability Samples Theoretical Sampling, uses selection of units based on an identified characteristic. Common in qualitative research, but not in quantitative. It is selection not stratified random Inferences from non-probability samples do not support generalized knowledge claims. Scope conditions are the characteristics which define the subject of the research. Universe , a conceptual grouping determined by scope conditions.