Only a third of the valid reports of sexual crimes against children are apparently prosecuted in Idaho, and just 16 percent of those successfully prosecuted are imprisoned, a study by the state Division of Family and Children's Services shows.

The survey of court action against alleged child sexual abusers during 1986 and 1987 showed a statewide conviction rate of over 80 percent. But it also showed that 60 percent of those convictions were the result of plea-bargains reducing the charge itself or the number of charges in total or promising therapy rather than prison.While three of every four cases resulting in a conviction start out with charges that could result in life prison terms, the study said only four of those 256 offenders are actually serving life sentences.

"Suspended sentences and orders for withheld judgment in favor of a period of probation are the fate of most prison sentences," the report said.

The report, intended to gather information on how the Idaho system handles child sexual abusers to help develop new strategies dealing with the crime, was issued just four months after the end of a 1988 legislative battle over the issue. The Republican legislative majority rejected the bulk of Democratic Gov. Cecil Andrus' "get-tough" package on grounds that it was poorly thought out and would have the opposite effect on prosecution of child sexual abuse cases.

The study said the focus of the court system in child sexual abuse cases is on modifying the behavior of the offender. But it said there were serious questions about uniformity and effectiveness of treatment programs around the state.

"There is the lack of current and accurate information and education in the professional research findings for treating sexual deviance," it said.

The survey showed that during the two-year period reviewed, the state Department of Health and Welfare received nearly 2,900 reports of child sexual abuse. Officials estimated that half those reports were valid, but of the more than 1,400 valid reports, less than 500 were taken to court.

Of the 474 cases taken to court, 34 were still pending at the close of 1987 and another 85 had been dismissed for various reasons, leaving only 355 convictions. That was less than a quarter of the valid abuse reports.

Ninety-eight of the convicted offenders received probation immediately, and another 55 were placed on probation after successfully completing the sex-offender program at the minimum-security prison in Cottonwood.

Only 56 were imprisoned, and less than half of them were serving sentences longer than five years.

The remainder were generally given county jail sentences or placed on work release.