Ball State University’s Center for Business and Economic Research says more than half of Indiana districts can’t operate efficiently because they’re too small.

The study found that districts with less than 2,000 students performed worse on tests than larger districts. Researchers say the performance gap may be because smaller districts can’t afford to use as many resources.

The study also found that smaller districts are less likely to offer calculus, physics and Advanced Placement courses.

The study recommends combining small districts. More than 150 had less than 2,000 students in 2014.