The police force of a large city had to decide between two different approaches to keeping the officers on the force informed about the changes in laws. An enlightened administrator of this force decided to put the two approaches to test in a research study. She decided to do a true experiment and assigned 30 officers randomly to each of the two programs for a period of 6 months. At the end of this time, all the officers who successfully completed the training under the two approaches were given a final test on their knowledge of the law. The 20 officers who completed Program A showed a reliably higher mean score on this test than did the 28 officers who completed Program b. The administrator wisely chose not to accept these results as decisive evidence of the effectiveness of the two programs.

A. Using only the information presented in this summary, explain why the administrator made her decision. Specifically, what threat to internal validity did she identify?

B. How might her decision differ if only 20 officers in each group completed their program (from the original 30 assigned to each), and there still remained a sizable difference favoring Program A?

C. Can the administrator say that either program is effective?