Just think about how a police department would estimate how many people are in a gang. The National Youth Gang Center's 2002 survey (pdf) reports there were 21,500 gangs and 731,000 gang members in 2300 US cities, less than previous years.

But how do police departments count their gang members? Are they counting "card-carrying" gang members who have undergone initiations? Do they count those who hang out with a gang, but haven't undergone initiation? How can the police tell who has undergone an initiation or not? Should they count "wannabes" who hang out occasionally on a gang corner? Should they trust information from police informants who are trying to save their own skins?

Police and DAs often have self-interest in making a gang problem seem larger than it is, in order to secure funding and legitimacy. In other cities, such as Detroit for many years, police and city hall simply denied any gang problem existed. So how do you count Detroit's gang members if they aren't even there?

Look at these ridiculous — and racist — estimates, documented by Irving Spergel in his definitiveThe Youth Gang Problem (1995).

In Los Angeles, police identified almost HALF of all black men between the ages of 21 and 24 as gang members! In Chicago, in 1988 gang squad officer Lawrence Bobrowski estimated the Windy City had as many as 120,000 gang members, a truly "windy" estimate! Not to be topped, LA District Attorney Ira Reiner in 1991 estimated that Los Angeles had 1000 gangs and 150,000 gang members!

To demonstrate the absurdity of these estimates, Spergel and Curry surveyed law enforcement in all cities across the US in 1988, and police in their survey reported only 320 gangs from LA County and 128 from Chicago. Curry and Spergel's "reasonable estimates" of actual gang members in ALL big cities in the US was less than 250,000 — or fewer than law enforcement estimates of gang members in Chicago and LA alone!

While there probably are more than half a million gang members in the United States, police estimates of gang membership are unreliable and influenced by organizational factors.