An algorithm that predicts number of prisoners from percentage of 10/11-year-olds that cannot read

From today's Guardian - an article by Neil Gaiman:

http://www.theguardian.com/books/2013/oct/15/neil-gaiman-future-libraries-reading-daydreaming

I was once in New York, and I listened to a talk about the building of private prisons – a huge growth industry in America. The prison industry needs to plan its future growth – how many cells are they going to need? How many prisoners are there going to be, 15 years from now? And they found they could predict it very easily, using a pretty simple algorithm, based on asking what percentage of 10 and 11-year-olds couldn't read. And certainly couldn't read for pleasure.