MAYOR BARRY'S decision to abandon the standard testing procedure for selecting new police officers is a mistake. The test is the method that wa chosen by the mayor's personnel department to select the best recruits for the police department. Now that over half the people who passed the tet turn out to be white -- while nearly 80 percent of those who took the test were black -- the mayor is suddenly changing the rules of the game. He proposes to lower the passing score, and, instead of offering the recruits with the best scores the first available jobs, he says he will use a lottery system to determine the order in which jobs are offered to those who pass the less-demanding test.

Mr. Barry made this sudden change because he said he fears that the city would be open to a discrimination suit otherwise. But this type of test has already been challenged in the Supreme Court and withstood charges of being discriminatory. Other challenges to the test, under different laws, are possible, of course, but this seems a slim excuse.

Faced with an unusually high number of whites doing well on the test, the mayor took the easy way out by lowering the passing score. In doing so, he also threw out the notion of merit. His decision did not so much help the racial balance of the group of recruits as it damaged the police department's image and race relations, as well as the confidence of its black recruits.

What if the mayor had honored the test results and the standard procedure? According to police officials, he would have found that only three-quarters of the 200 available jobs were filled. Starting from scratch to fill the remaining jobs, the mayor would have been able to say that there was a need to fill most of those slots with black officers, and to state his intention to give preference to blacks in whatever scheme he created to determine who would get those jobs. He might also have made a point of recruiting black candidates from the military and other police departments. As things stand how, however, the mayor has made a mistake. It is a mistake that reminds us of Police Chief Maurice Turner's answer when he was asked if the test was discriminatory. The chief, who is black, smiled and said: "I passed it."