Arbitron, a national radio audience-ratings company, is revising its recently released ratings for the Rhode Island market after discovering that questionable data was used to establish those ratings.

Arbitron said the ratings of WPR0-AM 630, the state’s top-rated radio station, may be “substantially affected” when the new ratings are released Monday.

The ratings information is used by radio stations to set rates for advertisers as well as to analyze what is attracting listeners. Arbitron uses “radio diaries” kept by selected individuals in the region who report their listening habits. Arbitron doesn’t allow anyone affiliated with the media to respond to the diaries, and media outlets typically have policies that ban their employees from participating in the market research.

A spokeswoman for Arbitron said yesterday that it had learned that six of the diaries used in the spring 2008 data for the Providence-Warwick-Pawtucket market area came from a household where at least one person is affiliated with the media. The media outlet was not named. Those six diaries were out of 2,160 diaries used for the total metro area of 1.37 million people. Each of those diaries counted as 640 listeners, said Arbitron spokeswoman Jessica Benbow.

WPRO-AM was the highest-rated station in the metro area in April, May and June. Arbitron began investigating its data July 29 –– four days after the report was released –– at the request of Clear Channel Radio, owner of WHJJ-AM-920, one of WPRO’s prime competitors.

Bill George, WHJJ’s program director, said he asked Arbitron to investigate after noticing an unusual jump in the ratings for WPRO, specifically in listeners 25 to 34 during the 6-to-10-a.m. slot, when WPRO’s John DePetro is on the air.

George said his station noticed WPRO’s rating for women in that age group went from 0 percent in the fall to 12.1 percent in the spring. The ratings for men in that age group jumped from 1.7 percent to 14.1 percent. The figures are percentages of all the people in that age group who are estimated to be listening to any radio station during that time period.

DePetro’s ratings rose from 11th place to 4th place overall in Rhode Island, George said.

In a letter he is sending to Clear Channel’s advertisers, George said: “The Arbitron crediting department identified six diaries in East Greenwich, RI, in the same household, from six people [three women and three men, ages 27-34], representing over 109 hours of listening during one week to WPRO between 6 a.m. and 10 a.m.”

Neither Barbara Haynes, general manager of Citadel Broadcasting, which owns WPRO, nor DePetro returned calls seeking comment.

Radio consultant Holland Cooke said “The spring survey is the most important survey for every single market, because much of Christmas [advertising] will be bought from and much of early 2009 [advertising] will be planned for based on that number.”

amilkovi@projo.com