You are asked to evaluate four studies that look at the relationship between participation in a jobs training program and wages earned after the program. Which regression has the highest level of internal validity?
A. The researchers took a random sample of households in the state and regressed wages on whether the individual participated in the jobs training program.
B. The researchers took a random sample of households in the state and regressed wages on whether the individual participated in the jobs training program, as well as gender, race, and previous job experience.
C. The researchers took a sample of men between the ages of 18 to 22. They randomly assigned half of the men to the job trainings program, and then regressed wages on whether the individual participated in the jobs training program.
D. The researchers took a sample of men and women between the ages of 18 to 22. They assigned the men to participate in the job trainings program, and then regressed wages on whether the individual participated in the jobs training program.
Answer: C. The researchers took a sample of men between the ages of 18 to 22. They randomly assigned half of the men to the job trainings program, and then regressed wages on whether the individual participated in the jobs training program.