The Average of Noise is not Signal, It’s Junk! More on NJ SGPs
Posted on February 3, 2014
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I explained in my previous post that New Jersey’s school aggregate growth percentile measures are as correlated with things they shouldn’t be (average performance level and low income concentrations) as they are with themselves over time. That is, while they seem relatively stable – correlation around .60 – it would appear that much of that correlation simply reflects the average composition and prior scores of the students in the schools.
In other words, New Jersey’s SGPs are stably baised – or consistently wrong!
But even the consistency of these measures is giving some school officials reason to pause and ask just how useful these measures are for evaluating their students’ progress or their school as a whole.
There are, for example, a good number of schools that would appear to jump a significant number of percentile points from year 1 to year 2. Here is a scatterplot of the schools moving from Over the 60th percentile to under the 40th percentile and from under the 40th to over the 60th percentile.
That’s right West Cape May elementary… you rock … this year at least. Last year, well, you were less than mediocre. You are the new turnaround experts. Good thing we didn’t use last year’s SGP to shut you down! Either that, or these data have some real issues – in addition to the fact that much of the correlation that does exist is simply a reflection of persistent conditions of these
That’s right West Cape May elementary… you rock … this year at least. Last year, well, you were less than mediocre. You are the new turnaround experts. Good thing we didn’t use last year’s SGP to shut you down! Either that, or these data have some real issues – in addition to the fact that much of the correlation that does exist is simply a reflection of persistent conditions of these 
