I finally obtained some data from DCPS on the demographics underlying the 2010-11 IMPACT scores. It’s difficult to draw strong conclusions from a one-year snapshot of the 4,000 educators who were evaluated. But it appears that, at least through the prism of IMPACT, the best teachers improve quickly in their first few years on the job and then hit a kind of plateau.

You can see the bar charts here.

The percentage of teachers with “highly effective” ratings increases with every year of experience up to year five, topping out at 28 percent. After that it’s up and down, peaking again at 26 percent in year nine before bottoming out at 15 percent in years 19 and 20.

The age chart is interesting because it shows a moderate hollowing out of the teacher corps at mid-career. There are about half as many teachers between the ages of 45 and 49 as there are in the 25 to 29 bracket. Seventy percent of the WTU bargaining unit is split evenly between the high and low ends of the age scale: 36 percent between 20 and 34, and the other 36 percent 50-plus. Twenty-eight percent are ages 35 to 49.

The age data also suggests that the young may have an easier time navigating IMPACT. The highest percentage of effective teachers (76 percent) is in the age group 20 to 24. Only 65 percent of teachers ages 60 to 64 were rated effective, a rate that drops to 62 percent for those 65 and older. Hard to say what exactly this proves, although it’s possible that principals and master educators might cut inexperienced teachers slack that is not extended to senior educators.

Both charts depict the brutal attrition rates common to most big city public school systems. In 2010-11 there were 667 teachers with two years experience in DCPS but 296 with three years service. The number continues to dwindle to year nine, which has a cohort with just 47 teachers. By age, the 25 to 29 group is the most heavily represented (666), and then dropping to 341 between ages 40 and 44.

I also got responses to a couple of other long-standing questions.What happened to teachers who were highly effective in 2009-10 but did not repeat in 2010-11?

There were 595 highly effective teachers in 2009-10 who remained in the WTU bargaining unit for 2010-11. Of those, 359 (60 percent) repeated as highly effective. Another 229 (38 percent) dropped to effective, and seven to minimally effective.

My other question: What happened to the 528 teachers found minimally effective in 2009-10? In 2010-11, 288 (54 percent) became effective while 142 (27 percent) remained minimally effective for a second year and were subject to dismissal. It means that DCPS probably had some success in coaching up teachers who were struggling. Seven teachers (1.3 percent) made the leap from minimally effective to highly effective.