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This dispute is just one example of the mathematical acrobatics required to isolate the effect of one teacher on their students' test scores, when so many other factors inside and outside the school's walls affect how students perform.

When a teacher whose students do well on tests moves to a school where test scores were improving the previous year, and average scores continue improving after that teacher arrives, it is hard to know how much of that continued improvement is due to the new teacher and how much to other factors.