In order to determine the effects of a new high carbohydrate diet intended to treat depression, a researcher conducts a study. Five hundred inmates in the Middletown prison are selected to participate in the study. The researcher puts the 400 inmates not in solitary confinement on the new diet. The 100 inmates in solitary confinement are given their regular meals. The 400 inmates on the new diet are not aware that they being studied or that their diet has been modified, but have many fewer reported cases of depression after 30 days.
The results of this study are likely to be invalid mostly because
◦ the treatment group and control group were not the same size.
◦ the subjects did not know that they were getting a treatment.
◦ the subjects did not volunteer to be treated.
◦ the presence of solitary confinement could be a confounding variable in this study.
◦ none of these