The limits of value-added

With all the plans to evaluate teachers by student performance gains — all relying on “value-added” analysis of test scores — we need to understand the limits of value-added to determine “good teaching,” writes Rick Hess.

For one thing, it’s only possible to generate value-added data for 25 percent to 30 percent of teachers.

As schools get smarter about rethinking staffing and integrating virtual instruction, it’ll be increasingly difficult to attribute a student’s performance to a single teacher (this is already a thorny question for districts where students receive substantial pull-out instruction or work with a reading coach).

Reading and math tests are reliable for “academically at-risk students who historically haven’t been taught the basics,” Hess believes, but may not reflect what advanced students are learning.

Now, we can measure teacher performance in various ways other than value-added scores. Such measures, along the lines that Tom Kane’s shop is piloting at the Gates Foundation, include familiar approaches like observation and more novel efforts like student feedback. Importantly, however, the validity of all of these techniques is being gauged by how tightly they correlate with grades three to eight reading and math scores. In other words, whether these are good measures of student learning is being gauged by how tightly they reflect value-added calculations. So, if our tests are flawed or are not capturing what we really care about, the proxy measures will be flawed or off-key in similar ways.

Despite its flaws,  value-added measures of teacher effectiveness are far better than the status quo, Hess argues. But “we should be careful how we use it.” And aware that critics are making valid points.

I’m not sure we’re ready to use student scores to evaluate teacher performance, but I know many teachers don’t trust principals to evaluate their competence.

Bryan Hassel has more on designing teacher incentives.

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  1. Erin Johnson says:

    Value added measures of teacher effectiveness are just as bad as the status quo. For a measure to be worth spending money on, it had better be both valid and reliable, something that both the VAMs for teaching nor the current evaluations of schools are.

    Substituting one horrific measure for another does not make the situation better.

    Certainly the Board of Testing and Accountability cited grave concerns with RttT using VAMs as their measure of accountablity.

    “…BOTA has significant concerns that the Department’s proposal places too much emphasis on measures of growth in student achievement (1) that have not yet been adequately studied for the purposes of evaluating teachers and principals and (2) that face substantial practical barriers to being successfully deployed in an operational personnel system that is fair, reliable and valid.”

    In plain English: VAMs will not work to reliably evaluate teaching. So why are we wasting this tremendous amount of RttT money in developing these data systems?


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