From “Mute the Messenger”, by Jason Stanford:
What he noticed was that most students’ test scores remained the same no matter what grade the students were in, or what subject was being tested. According to Stroup’s initial calculations, that constancy accounted for about 72 percent of everyone’s test score. Regardless of a teacher’s experience or training, class size, or any other classroom-based factor Stroup could identify, student test scores changed within a relatively narrow window of about 10 to 15 percent.
Stroup knew from his experience teaching impoverished students in inner-city Boston, Mexico City and North Texas that students could improve their mastery of a subject by more than 15 percent in a school year, but the tests couldn’t measure that change. Stroup came to believe that the biggest portion of the test scores that hardly changed—that 72 percent—simply measured test-taking ability. For almost $100 million a year, Texas taxpayers were sold these tests as a gauge of whether schools are doing a good job. Lawmakers were using the wrong tool.
Seems like a worthy subject for a blog named PreciseUncertainty.