A high school increased the length of the school day from 6.5 to 7.5 hours. Students who wished to attend this high school were
required to sign contracts pledging to put forth their best effort on their school work and to obey the school rules; if they did not wish to do so, they could attend another high school in the district. At the end of one year, student performance on statewide tests had increased by ten percentage points over the previous year. Does this improvement prove that a longer school day improves student achievement? No Yes