Necessity and Limitations of Randomization . Randomization is at the heart of experimental design. They are properly called quasi-experimental designs. The text says that experimental and control groups should be alike in all respects save treatment and that there are several ways to achieve such comparability. Randomization is one of them, but probability sampling and matching are suggested as others.
Probability sampling is equivalent to random assignment.
Taking two random samples from a population is the same as taking one larger sample randomly and then assigning the members of the sample to two groups. Each group is a random subsample of the larger group. This is a plus for the generalizability of the experiment, but it adds nothing to and takes nothing from the validity of the design for establishing causality. Matching is much like quota sampling, as the author notes. Matching will work quite well if one has identified all the extraneous factors and incorporated them into the matching profile.