The evaluation was to be done by taking the test scores of prior year and compared to the current year so that it is more fair.
For example, If a student scored at 20 percentile in 4th grade and then 40 percentile in 5th grade then the teacher is considered an effective teacher because the student improved compared to his peers. On the other hand, if the student scored at 90 percentile in 4th grade and then 60 percentile the next year then the teacher is deemed to be ineffective.