By announcing his intent to step down, Minneapolis schools Superintendent Bill Green put both central Twin Cities school districts in the market for new leaders. Green said he will not seek a second term when his contract expires next spring. And former St. Paul schools chief Meria Carstarphen left earlier this month to lead the schools in Austin, Texas.

Carstarphen and Green will have served St. Paul and Minneapolis three and four-and-a-half years, respectively, raising these key questions: How long does it take for a superintendent to have a lasting, positive impact on a school district, and how do leadership turnover and longevity affect staff and student outcomes?

Carstarphen came to St. Paul following a stint as a deputy superintendent in Washington, D.C., where she established a reputation as a talented, ambitious educator. Green is leaving the Minneapolis district to return to teaching and writing at Augsburg College. He served as interim superintendent for 17 months before accepting a three-year contract in 2007, and he's credited with bringing stability to the district following a tumultuous period in the leadership ranks.

The average urban schools superintendent has a tenure of about 3.5 years, according to 2008 data compiled by the Council of Great City schools, a national organization of the nation's 66 largest urban districts. That's a revealing and unsettling statistic.

Leadership stability, focus and follow-through matter in education. Excessive churn in the top job can disrupt a district's ability to make progress.

Not surprisingly, research on superintendent tenure compiled by the Denver-based nonprofit McREL research group found a positive correlation between longevity and academic achievement. District leaders who focus on the right goals, manage change effectively and stick around long enough to see results tend to have higher-performing students.

In a recent interview with the Star Tribune Editorial Board, U.S. Secretary of Education Arne Duncan said that he spent five years as Chicago's superintendent before being appointed to the national post. But he said that ideally a school CEO should spend seven to 10 years on the job to achieve meaningful academic gains.

And Geoffrey Canada, founder and director of the highly acclaimed Harlem Children's Zone, agrees that long-term leadership improves the chances for results, especially among inner-city kids.

In a speech here last month, Canada said he has worked on refining his comprehensive programs for nearly 20 years. Now the students who have been with Canada's program from birth to fourth grade have closed the achievement gap in New York between white and minority students.

Of course, other factors surely contribute to lagging academic achievement. Most research indicates that the major influences on student learning are home, family, community life and teaching quality.

But the fact that leader longevity can affect student performance is worth noting. As the Minneapolis and St. Paul school boards search for new superintendents, they'll look for strong, visionary leaders who can motivate staff and students. They will seek candidates with good political, managerial and communication skills. They should also look for candidates who are more likely to make longer-term commitments to the districts.