Apparently, sexual frustration increases the desire for alcohol, at least in fruit flies. Scientists randomly put 24 fruit flies into one of two situations. The 12 fruit flies in the ‘‘mating” group were allowed to mate freely with many available females eager to mate. The 12 in the ‘‘rejected” group were put with females that had already mated and thus rejected any courtship advances. After four days of either freely mating or constant rejection, the fruit flies spent three days with unlimited access to both normal fruit fly food and the same food soaked in alcohol. The percent of time each fly chose the alcoholic food was measured. The fruit flies that had freely mated chose the two types of food about equally often, choosing the alcohol variety on average 47% of the time. The rejected males, however, showed a strong preference for the food soaked in alcohol, selecting it on average 73% of the time. (The study was designed to study a chemical in the brain called neuropeptide that might play a role in addiction.)

(a) Is this an experiment or an observational study?

(b) What are the cases in this study? What are the variables? Which is the explanatory variable and which is the response variable?

(c) We are interested in the difference in means, where the means measure the average percent preference for alcohol (0.47 and 0.73 in this case). Find the difference in means and give the correct notation for your answer, using the correct notation for a mean, subscripts to identify groups, and a minus sign.

(d) Can we conclude that rejection increases a male fruit fly’s desire for alcohol? Explain.