A group of researchers want to test their hypothesis that sleep deprivation will decrease the number of lectures attended in a quarter. The researchers round up 350 students from University of Washington's 38,000 undergraduates and randomly assign each student to either 4 hours of sleep per night group or 8 hours of sleep per night group. Each participant's attendance is recorded for one quarter and the mean attendance days per group are calculated. Identify the independent and dependent variable and the population and sample.