In Pennsylvania, the ability of counties to assess property is all over the place, but it's rarely accurate.

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Not one of the 67 counties in Pennsylvania meets the standard of uniformity in determining market values for property set by the professional organization of assessors.

The standard is a ratio with the eye-glazing name "coefficient of dispersion." What it measures is how far the typical assessment is off the mark, either above or below the actual market value.

A county with a COD rate of 25 percent would have an average deviation of 25 percent, or to say it another way, the assessments can be 25 percent too high, or 25 percent too low.

Statewide deviations range from 53.8 percent for Greene County to 12.1 percent in Cumberland County. Allegheny County has a 22.3 percent deviation while Armstrong's is 34.7 percent, Beaver's is 40.5 percent, Fayette's is 30.7 percent, Washington's is 38.6 percent and Westmoreland's is 37.8 percent.

For assessors there are some communities in which it is easier to get the fair market value right than others.

Laura Foussekis, from the highly touted Maryland Department of Taxation and Assessment, said rural communities and those with old, unique homes tend to be harder to assess accurately than subdivisions with rows of similar tract housing.

Peter Davis, the chairman of the technical studies committee of the International Association of Assessing Officers, is a supervisor in the division of property valuation in the Kansas Department of Revenue where each county hires its own appraiser, but they each work off a computer-assisted mass appraisal system with oversight from the state.

Mr. Davis said his organization recommends that assessments keep to 10 percent deviation from the sale price. They are in the process of revising that standard so that older neighborhoods with diverse housing might have a standard of a 15 percent deviation but a tract of homes in a housing development should be closer to 10 percent.

He said the best way to handle assessments is with annualized updates. He discouraged a system of reassessment such as Pittsburgh experienced in 1998 when Saber Systems came in and reassessed the county.

"More often than not, taxpayers are not satisfied by the product," he said.

Instead he recommended a system of annualized reassessments. "In jurisdictions where they do annual appraisals they tend to do it in-house and build up an experienced staff," he said.

In Pennsylvania the statistics on the deviation can be found on the web site for the State Tax Equalization Board.

Elk County, which is largely rural, had a ratio of 44.2 percent even though it was reassessed just two years ago.

For instance, the last assessment in Lackawanna County was in 1973 and the assessments now deviate by about 50 percent. However, Montgomery County, which is on the northwest border of Philadelphia, has not reassessed since 1998 and has fallen within a 20 percent deviation ever since despite steady growth as rural areas are developed into suburbs of Philadelphia.

Thomas Brauner, the Montgomery County Assessor, said he can't explain why the numbers in his county come out better than elsewhere.

"The sales are what the sales are, there's no manipulation," he said.

Cumberland County's deviation at 12.1 percent was the best in the state after it reassessed just two years ago.

Mr. Davis noted that Allegheny County's ratio, at 22.3 percent based on the 2002 reassessment. has room for improvement.

"Twenty-two sounds a little high," he said.