Public school teachers in the District of Columbia are improving their performance because they’re motivated by the possibility of substantial pay raises or because they don’t want to get fired, according to an academic study of the groundbreaking teacher-evaluation system implemented by former schools chancellor Michelle Rhee.

The school system in the nation’s capital is unique among large school districts for the complexity of its teacher-evaluation system, which scores the performance of teachers based on classroom observations, student test scores and other factors.