A team of cognitive psychologists conducted a study on the effects of sleep deprivation on short-term memory decay. Forty-eight participants stayed in a lab for two days. Twenty-four of the participants were randomly assigned to a condition in which they were not permitted to sleep during that period. The other twenty-four were allowed to sleep whenever they wanted. At the end of the two days, the participants completed a task that involves reading a list of 20 words, then recalling as many words as possible. The appropriate statistical test is __________