Through the early 1990s and early 2000s, average scores on the SAT college entrance exam moved steadily upward. Now, for the past five years, theyve been drifting back down.

The reason? Unlike on the multiple-choice sections of the test itself, theres no one right answer. But a big factor is the larger, more diverse group of students taking the tests, combined with a widening scoring gap between the best-performing groups and those whose numbers are growing fastest.

Results released Tuesday show the high school class of 2009 earned a combined score of 1509 on the three sections of the exam, down two points from last year. The average reading and writing scores dropped one point each, while math scores held steady.

Experts caution against reading too much into the national average SAT score, given the test-taking pool changes over time and can vary widely among states. Still, the average score is now down nine points since 2006, when the writing section was first included and the test moved to a combined 2400-point scale.

Math scores are higher over the last decade, but reading scores are four points below their 1999 level.

The College Board, which administers the exam, emphasized the growing diversity of SAT-takers. Minorities made up 40 percent of last years group, and more than a quarter of the 1.5 million test-takers reported English was not their first language at home.

Fewer than half of high school graduates take the three-hour, 45-minute SAT, and the group is tilted toward higher-achieving, college-bound students. Experts generally pay closer attention to the National Assessment of Educational Progress, because, unlike college entrance exams, it represents the entire population of students.