The index measures each school's effort to challenge its students. The formula is simple: divide the number of Advanced Placement or International Baccalaureate tests a school gave in 2000 by the number of seniors who graduated in June of that year. Tests taken by ALL students, not just seniors, are counted. Only Washington area public schools that selected no more than half of their students through examinations and grades are included.

The rating is NOT a measurement of the overall quality of the school, but illuminates one factor that many educators and researchers consider important. Most U.S. schools restrict their AP or IB courses to their "A" students, denying millions of "B" and "C" students bound for college a chance to prepare themselves by taking a college-level course and test. A 1999 federal study showed that simply taking such courses is a better predictor of college success than good high school grades or SAT scores.

When compared with schools across the country, Washington area schools overall are among the most encouraging of AP and IB testing. In the list below, any school with a rating of 1.000 or above is in at least the top 5 percent of all U.S. high schools in encouraging students to take AP or IB tests. Also listed are the name of the school district and the percentage of a school's students whose family incomes are low enough to qualify for federally subsidized lunches and who also apply for that program. The number of subsidized-lunch applicants is a rough indicator of the socioeconomic factors that may affect a school's performance. The number of such applicants usually understates the portion of poor children in a given school.

Phelps High School in the District, like 41 percent of American schools, gave no AP or IB tests in 2000. The Thomas Jefferson High School for Science and Technology in Fairfax, with a rating of 6.055, is not included because it admits more than half of its students on the basis of grades and test scores. The list is designed to show how well schools are challenging average students. The ratings do not work well for schools like Jefferson that have no average students.

Flowers High School in Prince George's, Blake in Montgomery, Westfield in Fairfax, Forest Park in Prince William, Stone Bridge in Loudoun and Colonial Forge in Stafford are not listed because they are new schools that did not have full senior classes in 2000.

• 2001 Challenge Index By Washington Area School District