The authors synthesize findings from 219 studies published since 2000. These were culled from a systematic search of 4,800 empirical research studies and other literature. The 219 studies met the authors’ requirements for relevance and rigor. The authors also analyzed U.S. Department of Education data about principal characteristics from 1988 to 2016.

Among the studies in the literature review were six that used a statistical technique called value-added methodology to assess the impact of high-quality principals on student achievement. Some researchers question value-added methodology, saying differences in school-level student achievement that this methodology attributes to principals could instead be attributed to natural fluctuations unrelated to principals.