For each of the following studies, give the parameter of interest and state any assumptions that are necessary for the inferences to be invalid.
a. To investigate a possible link between jet lag and memory impairment, a University of Bristol (England) neurologist recruited 20 female flight attendants who worked flights across several time zones. Half of the attendants had only a short recovery time between flights, and half had a long recovery time between flights. The average size of the right temporal lobe of the brain for the short-recovery group was significantly smaller than the average size of the right temporal lobe of the brain for the long-recovery group.
b. In a study presented at a meeting of the Association for the Advancement of Applied Sport Psychology, researchers revealed that the proportion of athletes who have a good self-image of their body is 20% higher than the corresponding proportion of non-athletes.
c. A University of Florida animal sciences professor has discovered that feeding chickens corn oil causes them to produce larger eggs. The weight of eggs produced by each of a sample of chickens on a regular feed diet was recorded. Then the same chickens were fed a diet supplemented by corn oil, and the weight of eggs produced by each was recorded. The mean weight of the eggs produced with corn oil was 3 grams heavier than the mean weight produced with the regular diet.