What are the differences in achievement between students who have effective or ineffective teachers for several years in a row?
Scholars have expressed the variation among teachers in many ways. An early paper on value-added dating from 1992, for instance, suggested that teachers near the top of the performance curve in a district could get an additional year’s worth of growth out of students compared with the poorest-performing teachers. They have also found that the variation in teacher quality is primarily within schools rather than between them, meaning that almost all schools have both effective and ineffective teachers.
Despite much rhetoric from advocacy groups, the question of whether it’s possible to dramatically change outcomes for struggling students by assigning them to several effective teachers in a row remains an open one.
One of the most influential studies to assert this dates from 1996. Drawing from mathematics test data on Tennessee students linked to teachers over a four-year period, the study separated teachers by estimated effectiveness into quintiles. The researchers then tracked those students’ progression from 3rd through 5th grade.
They found that students assigned to three years of effective teachers outscored comparable students with three ineffective teachers by up to 50 percent points. The authors said that the effects of having several excellent teachers in a row accumulated over time.
More recent findings, however, have called into question the assumption that teacher effects can simply be added together year after year.
A number of researchers have highlighted the “fade out” or depreciation of teacher effects over time, a phenomenon that has appeared in a number of studies using value-added modeling. What this means is that impact of a teachers’ instruction on this year’s scores does not seem to persist when those same students move on to other grades. One 2008 study, for instance, found that teacher effects from one year had only half their impact in the next year.
Second, as noted earlier, estimates of individual teachers’ effects vary from year to year, and are more volatile for smaller cohorts of students or when fewer years of data are incorporated into the estimates.
Therefore, policies that pair the best teachers with underserved students would need to identify teachers who consistently produce strong gains and ensure that such gains compound over time, if those initiatives are to have a lasting effect on student scores. Such policies have not been tested at scale.
It can be said:
Some teachers produce stronger achievement gains among their students than others do. However, estimates of an individual teacher’s effectiveness can vary from year to year, and the impact of an effective teacher seems to decrease with time. The cumulative effect on students’ learning from having a succession of strong teachers is not clear.
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