The Chicago Sun-Times’ annual school rankings are based on average scores on 2008 state reading and math tests, not the percent passing state standards.

The Sun-Times uses a well-known statistical method called standardizing to analyze the “scale scores’’ of public school third- through eighth-graders who took the Illinois Standards Achievement Tests and high school juniors who took the Prairie State Achievement Exam this past spring.

The method compares each student’s score to the state’s average score and uses that information to create a school average that’s then compared to the average score of other schools.

Standardizing levels the playing field in years when one test might be harder to pass than others. It is unaffected by passing cutscore changes because it compares how a school scored relative to other schools, not to a state standard for passing. By using student scores, rather than percent passing, the method allows for more definition among top-scoring schools.

Only reading and math results were studied; they are the only tests that trigger sanctions under the federal No Child Left Behind law. Also, in each school, only grades in which at least 10 students were tested were analyzed.

The Sun-Times top-elementary list is based on schools that tested at least two grades in the third- through fifth-grade range. The top middle-grade list is based on schools that tested at least two grades, sixth through eighth.

Some schools, such as those serving grades K-8, might have won spots on both the elementary and middle-grade lists.

The rankings include percentiles, which are based on each school’s average reading and math score. The percentiles reflect the percent of students statewide who scored the same as or worse than the average student in the ranked school.

The analyses were done by Sun-Times staff reporter Art Golab.