Ex Post Facto Design:

One variation of the after only design is called the ex post facto design. This differs from the after only design because the experimental and control group are selected after the experimental variable is introduced instead of before. One advantage is that the test subjects cannot be influenced pro or con toward the subject by knowing they are being tested since they are exposed to the experimental variable before being selected for the sample.

Another advantage of this method is that it permits the experimenter to let the experimental variable be introduced realistically and to control only observations. This is useful in advertising tests which use commercial media.

If the experimental variable is such that exposure to it can be determined objectively on an ex post facto basis this bias of self selection can be eliminated and the design become essentially the same as the after only with control group design. In this latter case that ex post facto design may have a definite advantage over the other design, as the experimental variable will have exerted its influence in an entirely natural setting. An example would be a TV public service announcement designed to inform consumers about the pros and cons of nuclear energy. This could be broadcast over cable TV only and interviewers could then determine with some objective accuracy whether a home had cable TV or not.