Tuesday, February 7, 2012

The Logistics of "Thinning Out" Bad Teachers

Nick Kristof recently wrote another column calling for more high-quality teachers based on the latest paper on value-added measures of teacher quality.  There's a whole lot to discuss about both the column and the research paper, but let me focus for a minute on one small part of it.

Near the end of the column, Kristof writes that "If we want to recruit and retain the best teachers, we simply have to pay more — while also more aggressively thinning out those who don’t succeed. It’s worth it."  Recruiting, retaining, paying (and training, which is left out of this sentence) are all complex endeavors, but the "thinning out" part of the equation is often taken for granted.

Here's my question for Kristof: even if (and that's a big if) we can find a fair, accurate, and agreeable way to identify and dismiss the worst teachers, how many teachers are we actually going to dismiss in such a scenario?

The first question would obviously be whether we need to fire the bottom 5%, 10%, 25% or some other number.  That's up for discussion.

But the logistical question then, is how many teachers among the bottom X% 1.) can be readily identified and 2.) are planning on teaching again next year.  This will differ greatly by school and district, but in some places, this is going to be a very small number.Why?  Let's take a look at what the research says.

First, research consistently finds that it takes 3-5 years for a teacher to reach their potential.  So a good number of the lowest-performing teachers are simply going to be novices who will be better teachers next year.  We don't want to fire a first-year teacher who was in the bottom X% if we have reason to believe they'll be a really good teacher in a couple of years.  That would be incredibly counterproductive.

Second, research has consistently found that value-added measures of teacher effectiveness bounce around considerably from year to year -- particularly for teachers who teach a small number of students (e.g. a 4th grade reading teacher with 18 students versus an 8th grade math teacher with 150 students).  At least one paper has found that averaging scores over three years provides a much better, and more stable, estimate of teacher performance than does any single-year estimate.

Third, a number of recent papers have found that, at least in the first few years, many of the least successful teachers exit teaching.  This makes sense -- if you start a new career and find yourself completely overwhelmed, you're not likely stay very long.

Fourth, teacher attrition is exceedingly high in many high-poverty schools.  The general consensus is that about half of urban teachers leave the field within their first 3 years.

So, what does this mean?  We probably don't want to fire a whole lot of teachers in the first 3-5 years of their career because a.) they're still learning and improving; b.) we can't be that sure who the worst teachers are anyway; and c.) a good portion of the catastrophically bad teachers are self-selecting out of the field anyway.  If we give discount the first two years, when teachers are still learning their craft, and then take three more years to compute accurate value-added scores, it would only be teachers who'd taught for 5+ years who would really be ripe for firing due to low value-added scores.

Which means that the main herd we're trying to thin is the teachers who've made it through those first few years, reached their potential, and for whom we have accurate value-added estimates.  But how many teachers is that?  When I looked at high-poverty NYC middle schools a few years, I found that in the average school, only 1/3 of teachers had 5 or more years of experience.

Let's say that we're very confident in our ability to recruit and retain teachers who are better than our current teaching force and so we decide to fire all below average teachers (a full 50%) -- which would be a far more aggressive plan than any I've seen proposed.  First, the majority of these below average teachers are novices who are still improving and for whom we don't have particularly good estimates of ability.  Given that the majority of struggling beginning teachers either improve or self-select out of the profession, let's estimate that 2/3 of all teachers in their first 5 years are identified as below average teachers.  This would mean that only 1/6 of all teachers in their sixth year and beyond are below average teachers.  And since only 1/3 of teachers are in their sixth year or beyond, this would mean that only 1/18 of all teachers would both have 5+ years of experience and be rated below average.  This is a little under 6% of all teachers.

The average school in my sample had 72 teachers.  So, that's the equivalent of firing four teachers.  And that's under an extremely aggressive scenario.  Besides, now that you've rid your school of the chaff, who, exactly, do you want to fire next year?  And if you want to argue that we could be more aggressive and fire some of the novice teachers, that would mean there'd be fewer low-performing experienced teachers (since teachers tend to be roughly equally effective pre- and post-tenure).  So, for now, let's stick with the assumption that, under an aggressive plan, we'd fire four teachers this year in the average school.

Now, other districts have far more experienced teachers.  And it might make more of a dent there.  But a good number of our poorest-performing schools and districts are quickly churning through teachers too fast for firing low performers to make much of an impact.  Certainly, we should make every effort to rid our schools of the worst teachers (by increasing the performance of, and/or dismissing the lowest performers) -- I don't think anybody seriously disputes that notion.  Or, at the very least, I don't think anybody serious disputes that notion.  But will firing the lowest performing 6% of teachers in high-poverty NYC schools make a difference?  It's possible.  But let's be reasonable -- it's not going to make much of a difference.

So, yes, let's work harder to rid our schools of the worst teachers.  But let's not pretend it will be easy to do.  And, perhaps more importantly, let's not hold our breath while we wait to see if that bullet is actually silver.  In most places, other problems loom far larger.


Roger Sweeny said...

I've always wondered how your third and first point interact.

In the spirit of simple numerical examples: pretend half of first year teachers are terrible and half are wonderful; half are zeros and half are tens. The average teacher quality would be five.

During the next four years, all the bad teachers quit teaching. The average quality of that cohort is now ten. It looks like the teachers have gotten much much better. In actuality, none of them have gotten any better. The cohort is just different; the worst half is no longer part of the data.

How much of the supposed increase in teacher quality in the first few years is just bad teachers dropping out? I would say, "I assume all the studies control for this" but I've been burned by such assumptions in the past.

Corey Bunje Bower said...

You're right to be skeptical -- to my knowledge, there has been no long-term, longitudinal study of returns to experience among teachers (e.g. taking the same cohort of teachers over 30 years and tracking each one's progress).

But, we also know pretty definitively from a number of recent studies that teachers *do* improve over their first 3-5 years (numerous recent papers have examined the same cohort of teachers over that length of time).

Attorney DC said...

Corey: Good points in your post. As a former teacher, I saw many people who weren't cut out for teaching simply leave - I also saw much staff turnover at lower income schools, where teaching was usually more difficult. Given the above, I agree with you that it's bizarre for the education policy pundits to focus so heavily on "weeding out" the worst teachers, which will only serve to increase "churn" in the teaching staff of these schools, and will likely have little if any positive impact on the schools overall.

Corey Bunje Bower said...

Here's one of the latest research articles examining teacher effectiveness and attrition in their first five years