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 this year with that for the same time period last year.

The chart and map this week compare single-family house and townhouse sales figures in Loudoun County from January through March 2004, with those for the same period last year, 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.

ON THE WEB

For up-to-date home sales and tax assessments, real estate news and community profiles, visit www.washingtonpost.com/realestate