Do disadvantaged 12th graders score the same as well-off 8th graders?

Reading Wendy Kopp’s new book, I came upon a frequently used factoid: that students in poor areas “who do graduate will read and do math, on average, at the level of eighth graders in high-income areas.” The footnotes cite 2005 NAEP data. Usually I see this equivalency used in terms of race: black twelfth graders performing at the level of white eighth graders.

When I was writing Tested, I tracked down the specific data so that I could use it to illustrate the achievement gap. I spoke with three people who work on NAEP. One, a communications specialist, was comfortable with this use of NAEP data. But two psychometricians there confirmed what I was worried about: that the scales for the two tests were not aligned so that you could compare students across tests. So I found other ways to illuminate the gap—and you probably should too, unless the NAEP team has changed the metrics to make this sort of comparison appropriate.