A quick overview of the article is presented on the University of Chicago Press site: Downloads are not the primary reason for the decline in music sales. "Researchers from Harvard and Kansas find that impact of P2P sharing on U.S. music sales is "statistically indistinguishable from zero".
The overview also quotes:
"We match an extensive sample of downloads to U.S. sales for a large number of albums", write Felix Oberholzer-Gee (Harvard University) and Koleman Strumpf (University of Kansas). "While file sharers downloaded billions of files in 2002, the consequences for the industry amounted to no more than 0.7% of sales."
The author compiled data on nearly 50,000 music downloads of popular songs (on pop charts) and across eleven genre from 2 major P2P servers. They then compared these with the same pop chart songs CD sales, "it is striking to see that more than 60% of the songs in our sample are never downloaded".
This underlines what many online users have lived first hand. If an album is good enough, reaching the pop chart, it will gladly be bought by fans."