A study involved 12 patients comparing the effects of two types of caffeine--coffee and caffeine pills. Each patient was randomly assigned to take coffee with caffeine or caffeine pills on the first night of the study. Then each person took either caffeine pills on the next night. Hours of weakfulness were recorded for each night. What is the explanatory variable, what is the response variable, and what is the carry over effect if any? Is this an example of a matched pair?