Random assignment vs. random sampling
SAT glossary · Course Foundations
Definition
Random assignment (who gets which treatment) licenses causal conclusions; random sam- pling (who enters the study) licenses generalization to the population. Neither substitutes for the other.
Random assignment vs. random sampling in plain English
These are two different — and easily confused — types of randomness in an experiment, and each one earns you a different kind of conclusion. Random ASSIGNMENT means the researcher randomly decides who gets which treatment; this is what allows you to make a CAUSAL claim (that the treatment actually caused the result). Random SAMPLING means the researcher randomly picked who gets included in the study out of the whole population; this is what allows you to GENERALIZE the results to that larger population. Having one does NOT give you the benefits of the other — a study can randomly sample people but not randomly assign treatments (so no causal claim), or randomly assign treatments to a non-random group (so no generalizing to everyone).
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