Research released Monday by the National Bureau of Economic Research suggests that student performance on tests may be related not only to knowledge gained, but time between significant tasks. The new research -- by Ian Fillmore and Devin G. Pope of the University of Chicago -- examined student performance on Advanced Placement exams. The AP final exams are not always on the same schedule, so students who take more than one AP exam have varying amounts of time between the tests. The study found "strong evidence" that having shorter time periods between exams resulted in lower scores on the second exam. Students who take two exams with 10 days between them are 8 percent more likely to pass both exams than those who take the exams one day apart. An abstract of the study may be found here.