Value added teacher evaluations -- all you ever wanted to know

This week the New York Times published teacher rankings of 18,000 New York city teachers.

The ratings, known as teacher data reports, covered three school years ending in 2010, and are intended to show how much value individual teachers add by measuring how much their students’ test scores exceeded or fell short of expectations based on demographics and prior performance. Such “value-added assessments” are increasingly being used in teacher-evaluation systems, but they are an imprecise science. For example, the margin of error is so wide that the average confidence interval around each rating spanned 35 percentiles in math and 53 in English, the city said. Some teachers were judged on as few as 10 students.