D.C. teachers union files grievance over erroneous evaluations Davis said the union's goal is to find out if more than 44 teachers - including any of the hundreds who have been fired for poor performance since 2009 - were affected by miscalculated IMPACT scores. "Until we have all of the information that we requested, we're not going to assume that no other teachers were affected," Davis said, adding that the union will "do whatever we need to do in order to achieve remedy" for teachers who received inappropriately low ratings. Chancellor Kaya Henderson called the grievance "absolutely baseless." "We've been transparent about the error and we've made corrections to ensure no teacher was adversely affected," Henderson said in a statement to The Washington Post. Unfortunately, the WTU seems more intent on playing politics than working with DCPS to support and grow our teachers. Our city deserves better. A neutral hearing officer will decide whether the grievance is warranted and the union's requests should be granted, union officials said. That decision would stand unless either party requests an outside arbitrator, in which case the arbitrator would make a final, binding decision. Unlike in many jurisdictions, the evaluations in the District are not subject to collective bargaining because of a law that gives D.C. school officials sole authority over how teachers are judged. Then-Chancellor Michelle Rhee introduced IMPACT in 2009 as a centerpiece of the effort to improve the city's long-struggling public schools. It was among the first evaluation systems in the nation to link teachers" job security and pay to student test scores, making it a flash point in the national education debate. Teachers receive one of five ratings: ineffective, minimally effective, developing, effective or highly effective. Those rated ineffective are fired, as are those rated "minimally effective" two years in a row or "developing" three years in a row. Supporters say IMPACT is essential for school improvement, ensuring that excellent teachers are rewarded and inferior ones dismissed. Critics see a misguided and unfair system that overemphasizes students" test scores. The debate has largely centered on IMPACT's use of test scores to quantify the "value" that a teacher adds to student achievement. That calculation - which involves a complex algorithm and accounts for 35 percent of a teacher's overall rating - was the source of the evaluation errors in the 2012-13 school year. Mathematica Policy Group, a contractor responsible for calculating the value-added scores, made a coding error that resulted in the 44 incorrect ratings, half of which were too high and the others too low. School system officials announced the errors in December. At the time, they said that they were seeking to compensate teachers who mistakenly received low ratings. Officials paid $15,000 bonuses to three teachers whose scores were revised upward and offered back pay and reinstatement to the teacher who was wrongfully fired.