In yet another effort to force a summer referendum on whether to change Fairfax County's government, the leaders of an anti-tax group said yesterday that they will try to prove that they filed enough valid signatures to put the matter to a vote.

County Registrar Monica J. Horan determined last week that the group, called Citizens for Sensible Taxation, fell 778 signatures short of the number needed to compel a special election. Horan said yesterday the signature validation effort cost taxpayers about $27,500.

Marcia P. Dykes, a co-founder of the citizens group, said that, in hopes of finding enough mistakes to take the petition over the top, she would recruit volunteers to review more than 8,000 signatures that workers in Horan's office disallowed. The bulk of those signatures were invalidated because they were duplicates, illegible or did not belong to registered voters, Horan said.

The group hopes to change the form of government through the referendum this summer, believing that will force a special election for a new Board of Supervisors this fall, one year before its four-year term ends. Some legal experts have said that the board cannot have its term shortened by a change in government.

The group says its ultimate goal is to persuade state and local lawmakers to limit increases in real estate taxes at 5 percent a year.

Because of state laws, all legal matters must be resolved by July 6 if there is to be a referendum this summer.

Dykes came to that conclusion after discarding another option, which was to refile all the petitions along with 1,900 additional signatures that have been collected and go through the signature validation process again. Dykes decided to abandon that idea after Horan said it would be impossible to complete the process by July 6.

Ten percent of the county's 405,888 registered voters, or 40,589 people, needed to sign petitions to force a referendum; Horan found that the group filed petitions with 47,857 signatures, but that only 39,811 belonged to registered voters.

Attorneys for Dykes said in a hearing yesterday before Circuit Court Judge Thomas J. Middleton that they thought their best hope for finding mistakes was to review 1,280 signatures that Horan's workers could not identify, primarily because they were illegible. Middleton set another hearing for June 28.