The 50 million dollar lie

Here is an example of such a scatter plot, this one showing the ‘raw’ score (for value added, this is a number between -1 and +1) for the same teachers in two consecutive years. Notice how it looks like someone fired a blue paint ball at the middle of the screen. This is known, mathematically, as a ‘weak correlation.’ If the value-added scores were truly stable from one year to the next, you would see a generally upward sloping line from the bottom left to the top right.