According to this article in the NYTimes (registration required), two economists claim that file sharing has a negligible effect on record sales.
The RIAA's response? They fault the study for relying on survey data, saying essentially that people can't be trusted to tell the truth in such matters. The RIAA adds that people who agree to have their behavior monitored are not representative of the population they're after. In short, thanks for trying, but there's no reliable way to get data on file traders. How convenient.