Not surprisingly, teachers who are successful with students in one year tend to be successful in other years; hence, measures of a teacher's performance in the past tend to be a good predictor of how well
future students assigned to that teacher will achieve.
Second, each
teacher's value - added score is computed from just one class of 20
to 30
students, a small sample of all the
students that
teacher might have in a given year or might be
assigned in the
future.