PROVIDENCE — Arbitron, the national radio audience ratings company, yesterday released its revised ratings for the Rhode Island market, dropping WPRO-AM 630, the state’s top-rated station, to the number-two spot overall, behind WCTK-FM 98.1 Cat Country.

WPRO’s ranking in the popular morning drive-time category when host John DePetro is on the air fell more precipitously, from fourth to ninth place.

WPRO was the only station whose ratings dropped substantially.

Arbitron revised its spring rankings after discovering that questionable data was used to establish them. In determining station rank, Arbitron issues “radio diaries” to selected local individuals who record their listening habits. Each diary counts as 640 listeners. The company does not allow anyone affiliated with the media to participate in its diary program.

When the spring results were released last month, WPRO-AM was ranked the highest-rated station in the Providence metro area. But days later, Arbitron began investigating its data at the request of Clear Channel Radio, owner of WHJJ-AM-920, one of WPRO’s chief competitors.

Arbitron spokeswoman Jessica Benbow said the company learned that six of the diaries used to collect spring broadcast data in the metro market area came from a household affiliated with an unnamed media outlet, which then prompted the revised rankings.

Benbow would not confirm yesterday whether WPRO was the station in question or whether any sanctions have been imposed against the outlet.

Barbara Haynes, general manager of Citadel Broadcasting, which owns WPRO, did not return a call seeking comment on the new ratings.

Radio consultant Holland Cooke has 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.”

DePetro did not return a call to his cell phone. The morning host is said to be on vacation this week. Former ABC 6 TV news reporter Jim Hummel filled in for him during yesterday’s broadcast, as he has done in the past.

cneedham@projo.com