Philadelphia's fourth- and eighth-grade students scored lower than those in other major cities on a national test, according to a study released yesterday.

Philadelphia was one of 18 cities to voluntarily participate in the Trial Urban District Assessment, a special administration of the National Assessment of Educational Progress (NAEP) test.

Unlike annual state exams, whose difficulty is set by state departments of education, the NAEP is a national, centralized test considered tougher than many states'.

Philadelphia's scores came in the bottom group - those cities that had scores below the national average in both grades. Other cities in this group were Atlanta, Baltimore, Chicago, Cleveland, Detroit, Fresno, Calif., Los Angeles, Milwaukee and Washington.

Of the 18 cities, only five had scores higher than the national average in both grades: Austin, Texas; Boston; Charlotte, N.C.; Houston; and San Diego.

The 2009 tests were the first time Philadelphia participated in the national study, said David Weiner, the district's chief of accountability.

Philadelphia's average score for fourth graders was 222 compared with 239 for the national average and 244 for Pennsylvania fourth graders who take the NAEP exam annually, and 265 for eighth graders compared with 282 for the national average and 288 for Pennsylvania.

Superintendent Arlene Ackerman insisted that city students take the national tests so Philadelphia could compare its progress to other major cities', Weiner said.

The national test cannot be compared to the PSSA, Pennsylvania's annual tests, Weiner noted, because they are scaled differently.

City students have made seven straight years of gains on state exams.

The national assessment used a point scale of 0 to 500. About 1,800 to 4,300 students were assessed in each of the 18 districts.