EVEN ALLOWING FOR all the discrepancies that can attach to law enforcement authorities' crime statistics, the latest findings do show that serious crime was down in the District of Columbia and the suburbs for the first three months of this year compared with the same months last year. This does little for all the victims of robbery, burglary, larceny, murder, rape, assaults and car thefts this year, but if the trend continues, it is obviously good news.

Like the findings, the explanations for the decline are many and just as imprecise. Police always cite tougher sentencing of convicted criminals and larger police forces as sure-fire methods of curbing crime, even if criminologists are not so sure of such an automatic cause-and-effect relationship. Just as pertinent, surely, is the fact that there are proportionately fewer people in the 15-to-29 age range, which is the group most likely to commit crimes.

Still, though the supply of 15- to 29-year-olds may dwindle, it is not likely to disappear. What is more controllable at large--and also cited by authorities as a factor in this decline--is the amount of neighborhood vigilance. Police credit neighborhood watch programs for some dramatic reductions in housebreakings. Prince George's County Police Chief John McHale says the addition of 100 officers since July 1981 was critical to a drop in crime in his jurisdiction, but he adds that neighborhood crime watches "have had a direct bearing on the decline in housebreakings," which are down by 30 percent for the three-month period.

Similar reports are coming out of Richmond, where citizen crime-prevention operations have gone into effect in many neighborhoods. There is a difference between a snoop and a good neighbor who watches sensibly for unusual or suspicious activity and reports it accurately. The difficulty here, too, is that on many blocks the supply of watchers also has diminished during the working hours. Still, cooperative efforts can be organized to demonstrate a visible and concerned presence in a neighborhood. And that at least may send a would-be criminal away. Area police are eager to assist these efforts. Their offers are worth taking up.