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 nonathletes.
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.
d. College students enrolled in music classes participated in a study to compare the teacher evaluations of music education majors and nonmusic majors (Journal of Research in Music Education). Independent random samples of 100 music majors and 100 nonmajors rated the overall performance of their teacher, using a six-point scale, where 1 was the lowest rating and 6 the highest. The nonmusic majors rated their teachers higher by an average of .33 point.