Experts are warning that some cities have underreported and downgraded crimes in their localities. This does not mean that the nationwide drop in crime is illusory, but the decline may not be as dramatic as appeared in earlier reports.

• Authorities in Philadelphia, New York, Atlanta and Boca Raton, Fla. have been accused of falsely reporting crime statistics.

• Philadelphia has had to withdrawn its crime figures for 1996, 1997 and for at least the first half of 1998 from the national system maintained by the Federal Bureau of Investigation, because of underreporting, downgrading and general sloppiness.

• In Boca Raton, with the knowledge of the police chief, a police captain systematically downgraded property crimes -- reducing the city's felony rate by almost 11 percent in 1997.

• The former police commissioner of Buffalo, N.Y., says the pressure on police departments to prove their performance through reduced crime figures -- with promotions and pay raises increasingly dependent on good data -- "creates a new area for police corruption...."

Experts say police officers in many cities had for many years not attached much importance to crime figures beyond the need to report them to the FBI. But that has changed in recent years as police officers have come to the conclusion that crime data are an important crime-fighting tool.