Here are three solutions to Assigment 1, for different data sets. The solutions are similar in some ways, but come to different conclusions, since the situations producing the data are different.
Solution on s99testh: Concludes that mice who aren't allowed to exercise live less long when they are allowed to eat lots of food, probably because they become overweight.
Solution on s99testg: Concludes that some mice allowed only 4 grams of food per day die early due to malnutrition. Mice allowed to eat more may die early due to obesity if they aren't allowed to exercise. Allowing exercise avoids the obesity problem, and also seems to have some additional benefit.
Solution on s99testa: Concludes that a negative association of exercise amount with lifespan seen in the observational data is due to lurking variables, principally the existence of two kinds of mice, perhaps differing genetically. Exercise actually has little or no effect on lifespan. Restricting the amount of food the mice can eat increases lifespan a bit, partly because it reduces the weight of the mice, but probably for some other reason as well.