The work by the Center for Constitutional Rights is the latest in a series of examinations of the police tactic defined by a Supreme Court decision from decades ago, Terry v. Ohio, which permitted officers to detain someone briefly based on “reasonable suspicion,” a threshold lower than the probable cause necessary for a formal arrest.

The issue exploded in New York after Mr. Diallo’s killing, when those who protested the shooting contended there was a pattern of racial profiling in stop and frisks. A study in 1999 by Eliot Spitzer, then the state’s attorney general, found that blacks and Hispanics were disproportionately stopped in relation to their involvement in crime and their share of the city’s population.

In 2001, the city enacted a law requiring the police to provide quarterly reports about the raw data to the City Council and settled a lawsuit, also brought by the constitutional rights group, requiring that plaintiffs be given more valuable raw data.

Reporting by the police has recently become more regular. On April 30, Mr. Browne said that in 2010 there were 149,299 stops through March 31, about 13 percent fewer than in the first quarter of 2009. So far, he said, the stops yielded 186 guns.

As the numbers come out, analysts and academics pore over them to gauge effectiveness.

In March, researchers from the Center on Race, Crime and Justice at John Jay College of Criminal Justice said that more data and “increased public discussion of this controversial policing practice” were essential.

“If the public does not have access to the data, in a format that allows the experts to identify important trends, then it harms the public discourse,” said Donna Lieberman, the executive director of the New York Civil Liberties Union, which successfully sued to get the raw data. “And that is precisely the situation that we are in.”

Particularly vexing to Jeffrey A. Fagan, a professor of law at Columbia University who studied the issue for Mr. Spitzer, is that few can say what happens once the “11 or 12 percent” of street stops that lead to an arrest or summons get to court.