Ad lit.org: What Do Reading Comprehension Tests Measure? Knowledge.

The federal No Child Left Behind law requires more testing of students, and has spurred some frantic and ineffectual test preparation in many schools, says the author, E. D. Hirsch, Jr. Reading tests must use unpredictable texts to be accurate measures of reading ability, but if you cannot predict the subject matter on a valid reading test, how can you prepare students? Hirsch says you can't, and, therefore, you shouldn't try. The only useful way to prepare for a reading test is indirectly by becoming a good reader of a broad range of texts, an ability that requires broad general knowledge."