Reading more frequently is shown to help increase students’ reading levels. In early September, in order to increase students’ reading levels, Happy Hills Middle School instituted a policy of offering a small gift certificate to the local mall for every ten books a student read. By the end of May, the school had given out five times as many gift certificates as it had students, but students’ average reading levels were slightly lower than those of the classes before them had been at the end of their years.
Which of the following, if true, best helps to explain the apparent inconsistency in the results of the school’s policy?