One study suggests that policies setting a minimum age for school dropout can affect high school dropout as compulsory school attendance policies are predictive of educational attainment. Angrist and Krueger (1991), in an analysis of a national sample using instrumental variables to estimate policy effects, demonstrate that individuals born early in the year start school at an older age and drop out after completing fewer months of schooling than their peers who begin school at later months. They estimate that 25 percent of the potential dropouts complete more schooling because of compulsory schooling policies for students below certain ages.