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The Academic Performance Index, or API, has been described as the "cornerstone" of California's efforts to hold its schools accountable for the education of their students.

The idea is simple: Students take standardized tests that are intended to measure their progress in math and English. The results of those tests are used to gauge whether they are receiving a quality education.

But an analysis by this paper of data collected by the state suggests that student performance on the API has more to do with family income than how many students are crammed into a classroom, how much a district spends per student, how much teachers are paid, or what percentage of students are still learning English.

The analysis looked at data for the 160 school districts in Los Angeles, Orange, Riverside and San Bernardino counties,