Bias Confirmed

Last year, I wrote about a study of Tennessee TVAAS scores conducted by Jessica Holloway-Libell. She examined 10 Tennessee school districts and their TVAAS score distribution. Her findings suggest that ELA teachers are less likely than Math teachers to receive positive TVAAS scores, and that middle school teachers generally, and middle school ELA teachers in particular, are more likely to receive lower TVAAS scores.

The findings, based on a sampling of districts, suggest one of two things:

1) Tennessee’s ELA teachers are NOT as effective as Tennessee’s Math teachers and the middle school teachers are less effective than the high school teachers


2) TVAAS scores are biased against ELA teachers (or in favor of Math teachers) due to the nature of the subjects being tested.

The second option actually has support from data analysis, as I indicated at the time and repeat here:

Holloway-Libell’s findings are consistent with those of Lockwood and McCaffrey (2007) published in the Journal of Educational Measurement:

The researchers tested various VAM models and found that teacher effect estimates changed significantly based on both what was being measured AND how it was measured.

That is, it’s totally consistent with VAM to have different estimates for math and ELA teachers, for example. Math questions are often asked in a different manner than ELA questions and the assessment is covering different subject matter.

Now, there’s even more evidence to suggest that TVAAS scores vary based on subject matter and grade level – which would minimize their ability to provide meaningful information about teacher effectiveness.

A recently released study about effective teaching in Tennessee includes the following information:

The study used TVAAS scores alone to determine a student’s access to “effective teaching.” A teacher receiving a TVAAS score of a 4 or 5 was determined to be “highly effective” for the purposes of the study. The findings indicate that Math teachers are more likely to be rated effective by TVAAS than ELA teachers and that ELA teachers in grades 4-8 (mostly middle school grades) were the least likely to be rated effective. These findings offer support for the similar findings made by Holloway-Libell in a sample of districts. They are particularly noteworthy because they are more comprehensive, including most districts in the state.

Here’s a breakdown of the findings by percentage of teachers rated effective and including the number of districts used to determine the average.

4-8 Math           47.5% effective                        126 districts

HS Math            38.9% effective                          94 districts

4-8 ELA              24.2% effective                      131 districts

HS ELA               31.1% effective                       100 districts

So, TVAAS scores are more likely to result in math teachers being rated effective and middle school ELA teachers are the least likely to receive effective ratings.

Again, the question is: Are Tennessee’s ELA teachers really worse than our Math teachers? And, are middle school ELA teachers the worst teachers in Tennessee?

Alternatively, one might suppose that TVAAS, as data from other value-added models suggests, is susceptible to subject matter bias, and to a lesser extent, grade level bias.

That is, the data generated by TVAAS is not a reliable predictor of teacher performance.

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