An Update on New Jersey’s SGPs: Year 2 – Still not valid!

Posted on January 31, 2014

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I have spent much time criticizing New Jersey’s Student Growth Percentile measures over the past few years, both conceptually and statistically. So why stop now.
We have been told over and over again by the Commissioner and his minions that New Jersey’s SGPs take fully into account student backgrounds by accounting for each student’s initial score and comparing students against others with similar starting point.  I have explained over and over again that just because individual student’s growth percentiles are estimated relative to others with similar starting points by no means validates that classroom median growth percentiles or school median growth percentiles are by any stretch of the imagination a non-bias measure of teacher or school quality.
The assumption is conceptually wrong and it is statistically false! New Jersey’s growth percentile measures are NOT a valid indicator of school or teacher quality [or even school or teacher effect on student test score change from time 1 to time 2], plain and simple. Adding a second year of data to the mix reinforces my previous conclusions.
Now that we have a second year of publicly available school aggregate growth percentile measures, we can ask a few very simple questions. Specifically, we can ask how stable, or how well correlated those school level SGPs are from one year to the next, across all the same schools?
I’ve explained previously, however, that stability of these measures over time may actually reflect