How is success measured? One definition employed in this Washington Post report is a category called Cluster 31.

The term "cluster" was coined by Claritas, L.P., a national marketing demographics corporation headquartered in Alexandria. Claritas has developed definitions for 40 life style clusters, including Cluster 5 -- Furs & Station Wagons (Fairfax County), Cluster 28 -- Blue Blood Estates (Potomac) and Cluster 37 -- Bohemian Mix (neighborhoods such as Dupont Circle).

Jonathan Robbin, the Claritas founder, says that clusters are derived from a sociological concept called human ecology. Different kinds of people flourish in different kinds of environments, he says, and similar people tend to congregate in neighborhoods, or clusters, that provide similar resources.

Claritas does not claim that it can predict individual behavior. It does claim that, at a neighborhood level, it can identify by cluster the likelihood of who will buy what consumer product, information potentially worth billions to marketers.

In studying the 1980 census, Claritas found some neighborhoods that had never before been recorded as statistically significant. They were nearby suburbs of large cities, populated overwhelmingly by the young, affluent and educated. They would have been absolutely unremarkable American Dream neighborhoods except for one thing: they were overwhelmingly black.

As a result, Claritas created a new life style cluster called Cluster 31: Black Enterprise.

Nationwide, a Cluster 31 neighborhood is 1.8 times as likely as the average for all races and classes to have household income in the $50,000 to $75,000 range, according to Claritas' director of research, Doug Anderson. A Cluster 31 neighborhood also has 6.1 times the black residents of this hypothetical national average neighborhood.

Further, a Cluster 31 neighborhood has 1.3 times the national average of people with some college education. It has very few households making less than $15,000 a year, very few mobile homes, and very few people of northern, southern, or eastern European ancestry. Its inhabitants are highly likely to travel by railroad, smoke menthol cigarettes and shop at a health food or gourmet store.

While local Cluster 31 neighborhoods vary in income and consumption patterns, they include Prince George's County communities such as Kettering, east of the Capital Centre; Fort Washington, on Indian Head Highway in the southern part of the county, and Takoma in the northernmost tip of the District.

In descending order, the American metropolitan areas with the highest number of Cluster 31 neighborhoods are Chicago, Washington, New York, Detroit and Los Angeles. If you knocked on doors at random in the Washington area, you would be 11 times as likely to find a Cluster 31 family as you would on average in America.

Washington also has more Cluster 31s, relative to the national average, than it has Cluster 5s -- Montgomery and Fairfax county-style, highly affluent, predominantly white suburbs.

Cluster 31 represents the most affluent black neighborhoods in America, but it by no means contains all of middle-class black America. In fact, 193,000 blacks living in families making more than $35,000 a year live in predominantly white neighborhoods in the Washington area, Washington Post research discovered. This is almost double the 109,000 who live in Cluster 31 neighborhoods.

Nonetheless, Cluster 31 is the demographic benchmark from which many other statistical indices in this report were developed.

Other terms used in this series and their definitions:Typical: Median. Median income is that point at which 50 percent of the population makes more and 50 percent makes less.

Average: Mean. Average income is determined by dividing the total amount of money made by the total number of people making it. For the purposes of a study like this, typical or median is thought to be a more reliable snapshot of the middle ground than average or mean.

Suburban. All jurisdictions defined in the 1980 Census as the Washington Standard Metropolitan Statistical Area that are not the District of Columbia. This definition is not perfect because, as has been shown, Tysons or 12 other emerging cities in suburban Washington have more jobs than population, making them about as urban as the District. Further, it excludes counties such as Howard and Anne Arundel, the statistics for which are collected as part of the Baltimore area, and Frederick, Stafford and Calvert, which have been included in the Washington metropolitan area too recently to make many of their statistics comparable. However, this definition is useful because it does describe the core of the metropolitan area in which the vast bulk of the black suburban middle class lives.

Educated: Measured by attainment -- number of years of school.