1. 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. Source: "Science Finds Fried Foods Not Harmful," Moneysworth magazine, November 24, 1975, p. 1.

2. When traveling, some motorists assume that a res- taurant has good food and reasonable prices if there   are a lot of trucks parked outside. What other observed char- acteristics of a restaurant might a traveler use in deciding whether to stop and eat?

3. In doing "observational studies" while driving through suburban neighborhoods during the summer months, home burglars look for potential victims who are away on vacation. What kinds of observation indicators would tend to tell the burglars that a family is likely away on vacation? What are some of the strategies that vacationing homeowners might use in attempting to fool potential burglars into thinking they are not away on vacation?