Another troubling study

From the EPI paper:

Because of the range of influences on student learning, many studies have confirmed that estimates of teacher effectiveness are highly unstable. One study examining two consecutive years of data showed, for example, that across five large urban districts, among teachers who were ranked in the bottom 20% of effectiveness in the first year, fewer than a third were in that bottom group the next year, and another third moved all the way up to the top 40%. There was similar movement for teachers who were highly ranked in the first year. Among those who were ranked in the top 20% in the first year, only a third were similarly ranked a year later, while a comparable proportion had moved to the bottom 40%.

What's really amazing here is that the authors of the fire-the-bottom-80-percent paper actually cite other work by Timothy Sass and yet manage to overlook this.