An article in last week's Maryland Weekly detailing the scores of Prince George's County students on the California Achievement Test said county students had been performing as much as a year below national averages on the older Iowa Test of Basic Skills, before officials replaced it with the California test. Nine and seventh-grade students were as mush as a year behind on the last Iowa test, but third and fifth graders were reading at or near national averages. Test scores for all grades improved immediately when the California test was instituted, but the change in tests was mandated by the Maryland State Board of Education, not county school officials.

Students in Prince George's County elementary, middle and junior high schools continue to read, spell and cipher at or near the national average, test scores released last week indicated.

The county began administering the California Achievement test in 1980 because officials believed the test series then in use was outdated. Prince George's students were scoring as much as a year below national averages on the old Iowa Test of Basic Skills. Performance on the standardized tests immediately improved after the change and systemwide scores began to reach national averages for the first time last year.

The tests, a series of largely multiple-choice exams in vocabulary, reading comprehension, language mechanics, spelling, mathematics computation and mathematics concepts, wre given to 6,679 third graders, 7,806 fifth graders and 9,025 eighth graders last October.

The tests are scored in such a way that the "national average" score is equal to the grade year and number of months of schooling the child has completed at the time the test is given. For example, the national average for the third-graders who were given the test in October, the second month of the school year, is 3.2.

Test results show wide variations between the schools. For example, third graders at 34 of 120 elementary schools performed five or more months (about half a school year) below the national average. Students were five or more months above average at only 12 schools. School officials blame the traditionally high rate of student turnover at schools predominantly fed by apartment dwellers for poor performance in some schools.

In the following listings, R stands for reading, L for language, S for spelling and M for mathematics. The Total is a composite of the four scores.