Washington — The just-released results of writing tests given nationally to eighth and 12th graders last year show that the average scores posted by students at those grade levels improved over 2002 and 1998, the last two times the tests were administered.

For both grades, however, much of the increase in the average scores was driven by substantial growth in the share of students showing they can write at or above a level classified as “basic,” according to the U.S. Department of Education’s analysis of the results of the 2007 National Assessment of Educational Progress writing assessment. The share of students performing at or above a level regarded as “proficient” has remained unchanged since 2002, although it was slightly higher than it was in 1998, the Education Department’s analysis says.

At both grade levels, girls continued to outscore boys by a substantial margin. And although the performance of black students has improved substantially, both black and Hispanic students continue to lag well behind white students.

The 2007 writing assessment was administered to about 139,900 eighth graders at 6,810 schools and about 27,900 12th graders at 660 schools. Each student was asked to take up two of 17 possible writing tasks intended to measure one of three types of writing: narrative, informative, or persuasive. —Peter Schmidt