When the U.S. Catholic bishops release a report Friday on every accusation of a priest molesting a minor between 1950 and 2003, it will be the first comprehensive, long-term study of child sexual abuse among any group of adults.

While that makes the report important, it won't tell anyone whether Catholic priests are more or less likely to molest minors than are public school teachers, sports coaches or Protestant ministers, said David Finkelhor, director of the Crimes Against Children Research Center at the University of New Hampshire.

Although it's not a model scientific study, the bishops have the only statistics, he said. Large organizations such as the Boy Scouts and Big Brothers-Big Sisters have been compiling reports over the years, "but they have never released that data."

"The truth is there really are no valid estimates of the number of abusers in the general population, or in some other profession or in some other denomination," he said.

That is why Nicholas Cafardi, dean of the law school at Duquesne University and a member of the bishops' national Lay Review Board that commissioned the report, believes the bishops should be seen as leaders, not villains, in the effort to eradicate abuse.

"They really are the first group in our culture to agree to look at themselves in the mirror and agree to release the results of that to the public," Cafardi said.

The Lay Review Board, which oversees the bishops' efforts to remove offending priests and prevent future abuse, commissioned the study by criminal epidemiologists at John Jay College of Criminal Justice in New York City.

The statistical report will not name perpetrators or analyze individual dioceses, though many dioceses are voluntarily releasing their data.

The John Jay report will be accompanied by a second report from the Lay Review Board that will place the statistics in the context of church culture. It is intended to address the question: "What was going on in the church to let this happen?" Cafardi said.

Some dioceses are making extra efforts to interpret the study.

The Catholic Diocese of Pittsburgh will air a 30-minute program at 7:30 p.m. March 4 on KDKA-TV to respond to the John Jay study, said the Rev. Ronald Lengwin, diocesan spokesman. Pittsburgh is waiting until Friday to release its data because it wants to place its report in the context of the national report, Lengwin said.

A year ago, Bishop Donald Wuerl expressed concern about whether the John Jay study would distinguish between sex crimes and inappropriate behavior that could lead to dismissal from ministry but that would not be deemed illegal. In 2002, he led the successful fight for the bishops to define "sexual abuse" so that it did not even require a priest to touch a minor to determine that the child had been sexually exploited.

Wuerl later said he was satisfied that Friday's report would distinguish between types of offenses.

There are good reasons for defining suggestive but nongenital behavior as sexual abuse, but it makes the bishops' data difficult to compare with statistics based on sex crimes, Finkelhor said.

The law makes important distinctions about behavior, but from a psychological point of view, it's sometimes difficult to differentiate, he said.

"There are some kids who are traumatized by what seems like a rather innocuous thing and those who experience more serious things who are less traumatized. When it involves a person of substantial power, such as a priest, you can't assume that it wasn't traumatic -- nor can you assume that it was."

If a CNN report based on a reported early draft of the study is correct and 4 percent of priests have been accused of molestation since 1950, the figure will be far higher than the less-than-1 percent estimates church leaders had cited but lower than the 5 percent estimate offered by some critics of the hierarchy.

As for the unanswerable question of whether child molestation is more common among priests than other helping professionals, that's not the most pertinent issue, Finkelhor said.

"Whether it's 4 percent or 7 percent, or whether it's more or less common among priests than teachers, the point is that it occurs at an unacceptable level among people among whom it shouldn't happen, and who enjoy considerable trust in the eyes of children," he said.

Noting the efforts that bishops have made to remove offenders and implement prevention programs, he said, "I don't think that the church would do anything differently if it turns out to be a lot more or less common among priests than among other people." Although the study will help the church, its value to social scientists is limited, he said. The data was apparently not collected using scientifically consistent definitions. For instance, some dioceses that already released their data used different methods of counting priests.

Altoona-Johnstown counted accusations against religious order priests and included religious order priests in its total of priests working in the diocese, while Greensburg did neither. Thus, when Altoona-Johnstown reports substantiated accusations against 2.5 percent of its priests, and Greensburg reports substantiated allegations against 3.2 percent of its priests, the percentages aren't based on the same calculations.

"And when they ask whether an accusation was substantiated, we know that standards for that vary enormously from diocese to diocese and even within the tenure of some dioceses," Finkelhor said.

Victim groups have criticized the study because the bishops searched their own files and submitted their own numbers. But no one has better numbers than the bishops, Cafardi said.

"No organization besides the Catholic church keeps employee records going back 50 years. Whenever people say that this is based on incomplete or doctored records, I disagree completely," he said.

"One reason that the church is in court so often is that we can subpoena a diocese and they can tell you where a priest was in 1955. If you subpoena a corporation, they will tell you that those records were destroyed in 1972."

Cafardi believes that a major lesson the hierarchy will learn from the study is that "our bishops have nothing to fear, and everything to gain, from trusting the laity to assist them in dealing with these problems."

He believes additional studies are needed, and hopes the John Jay study, which the bishops paid for, will spur foundations to sponsor more detailed examinations. He wants a closer look at the backgrounds of the perpetrators, at the environment in which the abuse took place, and whether there were any patterns that appeared to make some young people more vulnerable to abuse.

"The John Jay study looks at the perpetrators more than anything else," Cafardi said. "But even there, we don't have complete data. We didn't pin down what seminaries these priests were trained at. We didn't pin down the circumstances of their lives or whether they were abused as children. There is more work to be done."