The laws of economic gravity continue to pull the number of home sales in Pittsburgh into a downward spiral.

For the 15th month in a row, the number of home sales in May were lower than they were during the same month last year in the five-county region of Allegheny, Butler, Beaver, Westmoreland and Washington counties, according to statistics from RealSTATs.

The 2,426 home sales in May were 663 fewer than in May 2007. But despite the 21.5 percent drop in sales activity, the median home price jumped 4.7 percent across the region to $122,000.

"May's numbers show that the current market requires greater patience on the part of sellers, but that patience is paying off in the form of higher prices," said Daniel Murrer, vice president of RealSTATs.

RealSTATs reports housing market activity weekly to governments and firms in the legal and real estate industries.

According to RealSTATs, this month's 21.5 percent decline is the steepest drop this year, topping the 19.1 percent fallback in March and the 18.4 percent drop in January.

Allegheny, Beaver, Washington and Westmoreland counties saw increases in median price ranging from 2.7 percent to 10 percent. Butler was the only county to see a drop in median price. Real estate prices in Butler fell 16.3 percent from $184,000 a year ago.

While Butler County's median price dropped for the third consecutive month, median home prices there remain the highest of the five counties at $154,800.

Washington County's median price in May was $135,000, up from $132,000 a year ago. Allegheny and Westmoreland County home prices currently stand at $119,000 and $119,950 respectively, while Beaver County remains the most affordable of the five counties at $97,320.