Why crime statistics can be confusing

What do crime statistics say about the nation's juvenile crime problem?

When properly interpreted, experts say, crime statistics can pinpoint trends with great accuracy. But those numbers can't, for example, tell us how many crimes are committed, or just who commits them. Why?

* Many crimes go unreported; those reported don't always result in arrests.

* Police statistics don't say how many people arrested were found innocent.

* Surveys based on statements by crime victims aren't reliable since victims don't always judge age accurately.

* Improvements in detecting, reporting, and recording crimes can inflate statistics even if behavior does not change.

* States establish different cut-off ages for juveniles, creating confusion in efforts to compare juvenile crime rates.

Hidden factors often sway the numbers. Commissioner Edward M. Murphy of the Massachusetts Department of Youth Services explains how major improvements in his state's juvenile justice system created some frightening figures. In 1968, he says, only 6 percent of DYS's population was in for violent crimes. Today, that figure stands at 25 percent - an apparent rise in violent juvenile crime of more than 318 percent.

The key fact, not found in the figures: In 1968, 25 percent of those entering DYS had committed acts such as truancy or running away from home - crimes which do not apply to adults.

When Massachusetts, spurred by federal legislation, removed such children from its institutions, the whole profile of the DYS population changed. Today, children under DYS care are more likely to be true delinquents. Violent crime has not increased among Massachusetts youth, Mr. Murphy says.

The experts admit they share the blame for the widespread confusion about crime trends.

Statisticians, who bravely plot and chart humanity's progress and pratfalls (''You think the reports are boring? You should see the raw data!'') must learn to translate their findings for the novice, experts say.