Solomon Four Group Design

Taking further the discussion of pre-test post-test control group design, and analyzing its problems, "Solomon Four Group Design" must be considered.

This design has two basic benefits;

1. It checks many of the internal validity threats, by providing more control over variables. And,
2. Due to the inclusion of two extra groups, the confounding variables influence can be checked, if any.

Explanation of Solomon Four Group Design

We will first take four randomized groups, Group A, Group B | Group C and Group D.
Pretest will be applied to the first two groups only.
Treatment will be provided to Group A and Group C.
Followed by Post Test of all groups.

First the post-test of Group C and D will be compared with that of Group A and Group B, if the results are considerably different, it is right to assume that the pre-test has had an impact on the final result.
Second, comparison between Group B's pretest and Group D's post-test will establish if any external factor has caused the change.
Third, Group A and Group C post-tests will be compared. If the results differ, which should ideally not, we will know that the pre-test caused the change.
And finally, Group B and Group D post-test comparison will let us know, if without the "treatment", only pre-test has caused the change.

This method is not very common in usage due to its high cost, technicalities involved, and usually higher time constraint of dealing with four random groups.

I will appreciate if Sir can add further to it.