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New York

Charter school principal: Enrollment policies can skew scores

Two schools whose students have identical test scores would seem to perform differently if they have different enrollment practices, according to a chart produced by a city charter school leader. It's not only the teachers union that is arguing that charter schools' enrollment practices can influence their apparent test performance. Unlike district schools, charter schools can choose whether to replace students who leave. Charter schools that do not practice "backfill" can end up posting scores that make it look like their performance is better — or worse — than it really is, argues the founding principal of Harlem Link Charter School, Steven Evangelista. In the Community section, Evangelista explains that when schools opt not to fill empty seats, "survivorship bias" skews test scores toward the results of students who remain enrolled. The bias renders test scores meaningless, even dangerous, if the scores are not presented alongside context about a school's enrollment practices, he writes: Strong schools take the time required to plan, assess, and tweak new initiatives until they become standard operating procedures. The lack of information provided alongside scores obscures this type of growth, creating perverse incentives for schools to “push out” students who are low performers and to “quick fix” by whittling down large original cohorts to smaller groups of survivors, uncompromised by new admittees. Evangelista says Harlem Link replaces students who depart, knowing that test scores could be adversely affected, in order to keep its budget stable and fulfill its mission of serving needy students. Last year, he writes, the school got lucky: The students who left were, on average, lower-performing than the students who left the previous year, so the appearance of large test school gains was easy to come by. It's a phenomenon that the teachers union has been particularly eager to put onto the agenda. After the city released elementary and middle school progress reports for last year on Monday, the union distributed a fact sheet noting high student attrition rates at several top-scoring charter schools. At South Bronx Classical Charter School, for example, between 20 and 40 percent of students that originally enrolled left before they were tested, and no new students replaced them, the union pointed out.
New York

Survivorship Bias And The Hidden Costs Of Backfill

Out of 90 charter schools that administered the New York State standardized tests in both 2011 and 2012, Harlem Link had the eighth-highest average increase in English language arts and math scores. This score improvement was amazing, fantastic, even inspiring. And misleading — because of a small, relatively unknown factor called "survivorship bias." Survivorship bias is a statistical term for an indication that there is some hidden factor that excludes certain members of a data set over time — namely, part of a sample that was there at the beginning is no longer there at the end and does not count in the final analysis. The smaller subset of those who “survive” over time might be better off than the original whole group simply because of who stayed and who left, not any value added over time. Simply put, every year, at every school, some students leave, and their departure changes the profile of who takes the test from year to year. Sometimes high-scoring students depart. At other times, low-scoring students depart. If schools continuously enroll new students (and some don’t), the same factor impacts the student population for these incoming students. At the end of this piece I chart a hypothetical situation in which survivorship bias shows how a school can appear to improve while not actually adding any value simply by not adding new students year after year. In large systems, there is so much mobility that these student profiles tend to cancel each other out because of scale. For example, the student population appears relatively stable from year to year in the third grade in Community School District 3, where 1,342 students in 30 schools took the state English Language Arts exam in 2012. But in small student populations like the one at Harlem Link, where only 52 third-grade students took the 2012 exam, a few students entering or leaving the school with certain test scores can make a big difference. When the state department of education releases test scores each year, however, it does not provide this or any other contextual background information alongside the scores. I believe that this process penalizes, in the public eye, schools that continue to enroll students to replace those that depart.