A while back we published an editorial bemoaning the impact of teachers who didn’t know their subject matter. One response we received was from a high school teacher who, surprisingly, at least to us, claimed teacher quality didn’t matter much in what eventually happened to students.

It was troubling to get that comment from someone who at least claimed to teach in the secondary education system. Now there is proof that good teachers do indeed affect the amount their students can eventually expect to earn over the course of their career, thanks to a study by economist and Stanford University professor Eric Hanushek.

Hanushek cites research that follows students after they leave school and enter the labor force. The results suggest that one standard deviation increase in math performance at the end of high school translates into at least 10% higher annual earnings. And the difference compounds over a lifetime. Using present-value calculations, the average present value of income for fulltime workers between 25 and 70 years old is $1.16 million. So a higher performance in math by one standard deviation adds over $150,000 to that figure.

Hanushek uses this method to figure out what a better-than-average teacher is worth to students. He estimates a teacher who is a half-standard deviation above average would lead to a tenth of a standard deviation improvement in student cognitive skills. The improvement, in turn, would add $10,600 to their earnings over a lifetime of work. So a slightly better than average teacher adds over $300,000 to the earnings of a room of 30 kids. Teachers who are even more effective add more. And, of course, below-average teachers reduce the lifetime earnings of their students in a similar way.

Hanushek figures the one-half standard deviation difference in teacher effectiveness is conservative. Other studies have shown that there is a wide difference in learning even in similar classrooms at the same school. Some teachers year-after-year just get more out of their students than do others, and the gains can be large: Some studies have found good teachers can get 1.5-year’s-worth of academic achievement in their students where others only get a half-year of gains.

As an additional mind exercise, Hanushek computes the effect of replacing the bottom 8% of the teacher quality distribution with teachers who are just average. The results are illuminating in light of the poor performance of U.S. students in math and science, compared to those from South Korea, Finland, Singapore, Hong Kong, and Canada, all of whom scored higher in the latest Programme for International Student Assessment: Eliminating just these low-performing teachers would bring the U.S. to the level of Finland.

The problem is Hanushek’s scenario for improved education is unlikely to happen because of the way most teachers are compensated. Teachers now get bonuses for advanced degrees and for seniority, neither of which necessarily correlates with teaching skill.

But removing 8% of the least effective teachers would be practical in at least one sense: We have more teachers than teaching jobs. Large school districts routinely have thousands more applicants than they have teaching positions to fill.

Leland Teschler, Editor

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