In a typical nonequivalent groups design in education you might nonrandomly choose two 5th grade classes to be in your study. This is nonrandom selection. Then, you could arbitrarily assign one to get the new educational program and the other to be the control. Random selection is related to sampling . After all, we would randomly sample so that our research participants better represent the larger group from which they're drawn.
Random assignment is most related to design .
In fact, when we randomly assign participants to treatments we have, by definition, an experimental design . Therefore, random assignment is most related to internal validity . Purchase the complete printed text of the Knowledge Base online Research Methods Knowledge Base, 2nd edition.