ACCORDING TO the statistics, crime is down and it is down dramatically. The attorney general announced over the weekend that the number of serious offenses reported to the FBI by state and local law enforcement officials declined in 1983 by 7 percent. This was the largest drop in the figures since 1960, and the first time they have dropped two years in a row. Why the decline?

Sophisticated, and perhaps cynical observers will say that fewer crimes are inevitable simply because the numbers of people in the high-crime age group are declining. True, because of the falling birthrates, which began in the '60s, there are now far fewer young men in the 18-26 age group than there were five or 10 years ago. But is this demographic phenomenon the whole answer, or are other factors -- new policies, changing laws, things we can control -- at work here? It appears that they are.

In the mid-'70s, the federal Law Enforcement Assistance Administration began funding programs in state courts directed specifically at chronic offenders. These are the violent criminals who account for a share of crimes far out of proportion to their numbers, even in the criminal population. It is not unusual for a single such individual to commit hundreds of crimes each year before being caught. These same offenders often have a long history of lawbreaking and a couple of convictions, first leading to probation and then to prison. District attorneys are now trying to identify these defendants immediately, forgo plea bargaining and expedite their trials and, finally, ask judges for long prison sentences. Large numbers of the worst offenders, then, have been immobilized by incarceration.

The success of this approach can, in part, be measured by the steady increase in the prison population, which has gone up every year for the last 10. At the end of 1983, there were a record 438,830 inmates in the nation's prisons; in 1974 there were only 229,721. Many are chronic offenders sentenced to long terms while still in their early twenties. Given their records, if they were on the street, each could be expected to commit scores of new crimes each year.

The movement toward mandatory sentences has also had an impact. Forty-three states now require a prison term for certain offenses such as violent crimes, narcotics or offenses committed with a gun. In most states, and in the city of Washington, this has put a strain on prison facilities, which are operating at an average of 110 percent of capacity, but the public response has been to build more institutions, not to repeal the special penalty laws. During the past two fiscal years, state systems spent more than $900 million and issued $2.25 billion in bonds to construct and improve prisons.

Whether you agree with these new get-tough policies or not, statistics from a variety of sources lead to the conclusion that they are having an effect on the overall crime rate. The figures are too impressive to be dismissed as a reflection of either dumb luck or the falling birthrate.