Autism is a complex disorder usually not diagnosed in children until after age 3. It is characterized by a range of behaviors, including difficulty in expressing needs and inability to socialize. The cause is not known.

Scientists have been revising how common they think the disorder is. Past lower estimates were based on smaller studies. The study released Thursday is one of the first scientific papers to come out of a more authoritative way of measuring it.

"This is a more accurate rate because of the methods they used," said Dr. Eric Hollander, an autism expert at New York's Mount Sinai School of Medicine.

The study involved 2002 data from parts or all of 14 states - Alabama, Arizona, Arkansas, Colorado, Georgia, Maryland, Missouri, New Jersey, North Carolina, Pennsylvania, South Carolina, Utah, West Virginia and Wisconsin.

Researchers looked specifically at children who were 8 years old because most autistic kids are diagnosed by that age. The researchers checked health records in each area and school records when available, looking for children who met diagnostic criteria for autism. They used those numbers to calculate a prevalence rate for each study area.

Included were autism-linked conditions like Asperger disorder, which some experts say might partly account for the higher rate.

Dr. Fred Volkmar, director of the Child Study Center at Yale University, said the educational records researchers relied on in some states may be misleading. Sometimes, if a child has problems that seem like autism, parents will push for an autism label to get additional educational services, he said.

Rates varied dramatically among states, in some cases. The rate was 3.3 per 1,000 in the northeastern Alabama study area and 10.6 per 1,000 in the Newark, N.J., metro area.

Researchers say they don't know why the rate was so high in New Jersey. They think the Alabama rate was low partly because of limited access to special education records.

The study was not an effort to find the cause of autism, still a point of debate. While many advocacy groups blame the vaccine preservative thimerosal, scientists are putting more focus on possible genetic causes, according to a recent Stanford University study.

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