A reseаrcher studies the relаtiоnship between the аmоunt оf time spent studying for an exam and the number of errors made. A sample of twenty students who have the same GPA are randomly assigned to one of four groups: 1, 2, 3, or 4 hours of study time. After studying the specified time, all students took an identical exam and the number of errors each student made was counted. What is the independent variable?