Hull's study drew conclusions about hallmarks of effective school principals. The best of them are true instructional leaders who set the tone for school curriculum and teacher development, he said. Meanwhile, they retain their traditional administrative duties, which include setting the tone for student discipline.
"Good principals help teachers improve instruction," Hull said. "They identify those that have high skills and make sure they retain them. Those teachers that don't have good skills, they are able to replace with teachers who have greater skills. They are good motivational leaders who create an environment where students and teachers enjoy coming to school."
Hull said there is no "silver bullet" for solving high principal turnover, but several recommendations arise from current research.
"First, we have to identify highly effective principals," he said, adding that sophisticated value-added measurements like those used in the "School Leaders Matter" study should be part of a broader process for doing that. Rivkin, too, believes value-added evaluations are only a partial measure of principal effectiveness.
"Districts have to be very careful not to over-rely on value-added estimates as their sole measures of principal or teacher performance," he cautioned. "They provide one piece of information that should be used in combination with personal observation, a standard tool in virtually all occupations and industries."
Both researchers recommend that school districts create incentives for keeping excellent principals in one school long enough for them to make a positive impact.
That can be done through higher salaries, better working conditions and more support for principals who remain at the most challenging schools, Hull said.