The Norwegian study looked at almost 2,000 online music users, all over the age of 15. Researchers found that those who downloaded "free" music - whether from lawful or seedy sources - were also 10 times more likely to pay for music.
First off: those who download free music are 10 times more likely to pay for music than whom? It is impossible for those who download free music to be more likely to pay for music than online music users who pay for all the music they download. This leads me to conclude one of two things:
Either the study compared online music users who download free music to people who buy their music in a physical form (records, tapes, and CDs).
Or, the study actually concludes that people who download free music pay for 10 times more music than those who buy their music online, but do not ever download free music.
A long time ago, I read that people who made (and gave away) mix tapes were among the most prolific purchasers of music. They loved music. They actively sought out new music. My second interpretation - if that's what this study actually concludes - might indicate that those who buy the most music online also sample the most music online, from all sources.
However, this type of study is nearly useless without separating those who download lawful "samples" and those who download pirated versions instead. Is there even a distinction between those two groups? If so, does one group buy more music than the other?
If there is no distinction between the groups - if those who are likely to download music online typically download from both legal and illegal sources - can you separate them into a group that prefers free music from legal sources, a group that prefers free music from illegal sources, and a group that doesn't have a preference? Which group is largest, and how much is each group spending on music? Which is buying the largest percentage of music compared to the free music they download?
Without answers to those question, the Guardian's assertion "This would make music pirates the industry's largest audience for digital sales" is completely unfounded.
Ars Technica has a much more thoughtful article on the subject, but their analysis seems to be based on a Google translation of a Norwegian news article. Frankly, I'd still like to know more about the study. (First link via j-ko)