We decided to use NAEP to track college preparedness over the past two decades. This was easy for reading, since the "prepared" level is set at the same point as "proficient"--and it's a breeze to find the percentage of students at or above proficient since 1992. Math was harder both because its cut score fell between "basic" and "proficient" and because a new test and scale were introduced in 2005. We owe special thanks to the National Center for Education Statistics, which calculated the results for math that are displayed below using restricted NAEP data.

Here's what we found--the trends for "college preparedness" over time--since 1992 for reading and since 2005 for math [...]

The main storyline is consistency. The rates have bounced between 35 and 40 percent; they are currently up a bit since a low point in 2005. Considering that U.S. high school graduation rates are also up significantly over this period--and thus a greater portion of students are reaching the twelfth grade--these are mildly encouraging trends, despite the overall flatness of the lines.

Furthermore, the (mostly) flat lines are not due to "Simpson's Paradox." In education, that phenomenon explains why some aggregate trend lines look flat or worse, even though every student subgroup is improving, because of the changing demographic composition of the total student population (e.g., lower-scoring Latino students are gradually replacing higher-scoring white students). In this case, though, each of the three main subgroups shows basically the same flat trends:

Nobody should celebrate the fact that fewer than 40 percent of high school seniors are academically prepared for college-level work. (ACT shows similar "readiness" proportions for those who take its high-stakes test.) But why do we have the sense that this problem has worsened over time?

That's because the proportion of recent high school graduates attending college is far higher than the proportion of twelfth graders who are prepared for college--and that gap has worsened over time.