Yesterday, I noted that American Federation of Teachers (AFT) president Randi Weingarten cited an imaginary statistic on MSNBC’s “Morning Joe.” Weingarten claimed that “most teachers right now in America have less than two years of experience.” That’s clearly false because the most recent NCES data shows that 91 percent of government school teachers had more than three years of classroom experience in 2011-12.

As I noted in an update to my post, some claimed that Weingarten had probably intended to refer to the mode, not “most.” Weingarten herself later admitted that she misspoke and meant to refer to the mode, but even then, the data she meant to cite was out of date. What she said was technically true for 2007-08 (though misleading, as I will show), but she claimed that this was the case “right now,” which is false. In fact, the most recent data (see page 12) show that the mode for teacher experience was five years in 2011-12.

Nevertheless, she still claims that the statistic she meant to cite buttresses her point. Actually though, her use of that statistic is misleading.

Let’s say the AFT threw a party that eight adults and two children attended. Their ages were 45, 41, 39, 38, 37, 35, 34, 32, 1, and 1. When asked about the ages of the attendees, Weingarten reports, “the mode of the partygoers’ ages was less than two years old.”

That’s technically true, but also terribly misleading. Using the mode to answer that question without further context obscures the fact that only 20 percent of the partygoers were under age two and that the median age was 36.

Indeed, Weingarten’s (intended) use of the mode to describe the level of government school teacher experience was even worse than the above hypothetical, since only 9 percent of teachers had less than three years of experience (and it’s not clear from the NCES data how many of those teachers have less than two years experience, as Weingarten claimed). The average years of experience is 13.8 (page 10).

Weingarten intended to use the mode to support her claim that “you don’t have a lot of the people who are senior teachers any more.” It’s hard to know what she meant by “a lot” or “senior” but more than 57 percent of teachers have 10+ years of experience and more than 21 percent have 20+ years of experience. A third have been teaching for between three and nine years.

There is some truth to the broader point that the teacher workforce is “greening,” though not nearly to the extent that Weingarten implies. A Consortium for Policy Research in Education report notes that about 37 percent of teachers had less than 10 years of experience in 1987–88 and that number has climbed about five points to 42.3 percent in 2011-12 (according to NCES).

Teachers today may be slightly less experienced on average than 25 years ago, but with nearly six in ten teachers having more than a decade’s worth of experience under their belts, Weingarten’s use of statistics is misleading at best.

How concerned should we be about this greening? At this point, probably not very. As the Urban Institute reported in 2010, teachers show the largest productivity gains in the first few years in the classroom, “after which their performance tends to level off. At some point, it even declines:

This and other research shows that, on average, teachers with more than 20 years of experience are more effective than teachers with no experience, but are not much more effective than those with 5 years of experience (Ladd 2008). Studies have also documented some evidence that effectiveness declines after some point, particularly among high school teachers. In fact, evidence suggests that the most experienced (greater than 25 years) high school mathematics teachers may be less effective than their less experienced counterparts (Ladd 2008) and even their inexperienced colleagues (Harris and Sass 2007). [Emphasis in the original.]

In summary, the teaching workforce is only slightly “greener” than a quarter-century ago, but the vast majority (91 percent) have more than three years of teaching experience, which is around when productivity gains begin to level out. Presenting the mode for years of teacher experience without that context greatly distorts the reality of the teaching workforce.