A study was done to compare two diets to help diabetic male patients lose weight. Each diet reduced patients’ intake to 1,800 calories per day. In the control condition, patients chose their own meals from a list of allowed foods, but their calorie intake was not allowed to exceed the limit. In the treatment condition, designed to minimize fluctuations in insulin levels, patients’ meals came from the same list of foods but were planned so as to achieve specific percentages of complex carbohydrates and proteins. Forty men were randomly assigned to each condition. All participants monitored their insulin levels. The experiment lasted 6 months; during that time 15 men in the control condition dropped out of the study (e.g., due to difficulties managing insulin, complications associated with diabetes). Three men in the treatment condition dropped out of the study;

two moved out of state, and the work schedule for a third prevented him from attending appointments for the study. At the end of 6 months, the average weight loss for the 25 men who completed the control condition was 11 pounds, and 10 pounds for the 37 men in the treatment condition. The researcher concluded that the treatment, specifying the percentages of calories from different sources (e.g., complex carbohydrates, proteins), is not helpful for achieving weight loss among diabetic men.

A. Identify a possible threat to the internal validity of this experiment and explain how this problem could account for the results of the study.

B. Assume that for the 80 men in the study, a pretest measure was available that measured the degree to which they were able to regulate and correct, if necessary, their insulin levels. Describe how you could use these pretest scores to determine whether a threat to internal validity occurred.