Philadelphia — In the last 15 years the percentage of blacks on Philadelphia's police force has shrunk from 20.8 to a 17.5 as the city's black population has increased to nearly 40 percent.
This might seem like just more of the same from a police force President Carter's Justice Department called one of the most racist and brutal in the nation.
But here's a case where statistics are grossly misleading. In the nearly 21/2 years since William J. Green assumed the mayoral mantle from Frank Rizzo, the department has undergone radical change. Even the harshest black critics acknowledge that great progress has been made.
''The (motto) of the previous city administration was, 'My men right or wrong ,' '' says Donald Fair, the department's communications director.
''This administration says: 'My men -- and women -- when you're right, you're right, when you're wrong, you're going to have to pay the consequences.' ''
In fact, another Rizzo administration motto might have been, ''No communication with the press,'' at least on police matters, as I can attest from trying many times to interview police officials under Rizzo.
Now reporters are kept abreast of good news and bad. The department shares positive developments in minority hiring and the dramatic reduction in police use of deadly force. But it also communicates on officer firings or cases where police use excessive force in making an arrest.
Part of the reason for the open communications policy is the communications director himself. A local television reporter during the Rizzo administration, Don Fair was one of the police department's most vocal critics.
In the past two years Mr. Fair has written over 300 press releases. The number during the Rizzo's reign: zero. He has often worked well into the night to document facts for reporters, facts that often have not shown the department in the most flattering light.
The changes are more than just press-release deep, though much remains to be done. Among major positive developments:
* The number of people shot and killed by the Philadelphia police in 1981 was only four; this is down from 19 in 1979. The extremely high police use of ''deadly force'' was one of the main reasons experts considered the department one of the nation's worst.
* In 1979 women on the force were only working in six districts of the city, in limited roles; today, they are working in every area of the city, in the crime lab, in community relations, on the beat, and in the narcotics unit.
* A new citizens' complaint unit is required to answer within 45 days complaints against police officers or department procedures. There was no such mechanism before.
These changes have not only won the department greater respect but have helped ease racial tensions in the City of Brotherly Love.
In addition, the deputy police commissioner is black. So is the city's managing director, to whom the police commissioner (who is white) reports.
Robert W. Sorrell, president of the Philadlephia Urban League and one of the harshest critics of the police department in the past, praises the many positive changes.
But he remains deeply concerned that ''the Green administration has not . . . acted forthrightly to develop policies'' that would result in more minority police hiring.
One of the major stumbling blocks to hiring more blacks at this juncture is the fiscal restraint that Mayor Green has had to exercise just to balance his budget these days. Another is the tough, and minority advocates say unfair, tests that prospective police officers must pass.
The black community here wants a quota of openings set aside for blacks. Mayor Green says such a system would fly in the face of civil service laws. He wants those who do best on the test to get the first shot at openings on the force.
But he is moving to establish some kind of comprehensive ''pre-test training'' to prepare minority applicants for the tests.