The Washington Post regularly tracks housing sales and prices throughout the Washington area each Saturday in the Real Estate section, comparing information collected for each residential Zip code from each time period in 2001 with that for the same time period in 2002.

The chart and map this week compare single-family house and townhouse sales figures in Calvert County from January through September 2001, with those for the same period in 2002, showing the total number of sales, the median prices and the changes in the medians. The median is the point at which half of the sales prices were higher and half lower.

The sales and price information, collected by The Washington Post, is based on actual sales recorded in local government offices. That information is in the chart on the right. It excludes some types of transaction, particularly those that are not at market price.

As with any statistical compilation, the greater the number of transactions, the more reliable the statistical trend. Moreover, at any given time there may be more sales activity in certain market niches or price ranges, for instance starter homes or high-priced houses, than at other times, and this may influence a median price from year to year.

The map provides a snapshot of price trends throughout the geographical area covered by each week's information.

Some Zip codes cross county borders. The map of Zip code values shows the trend for the entire Zip code, including transactions outside the county. Those out-of-county transactions are not included in the table.