In a health study, the dietary habits of 420,000 men were examined over a 3-year period. For those who ate  no fried foods, the death rate was 1208 per 100,000 man-years. For men who consumed fried foods more than 15 times a week, the corresponding rate was only 702. Considering that the relative ages of members of these two groups were not mentioned, critique the internal validity of the experiment.