Apple has succeeded in reducing the size of lots of useful things, and as it turns out the number 22 is the key one facing the company's iTunes music service, and by extension other music download services and the entire universe of paid-for digital content itself.

Some 67.4m iPods had been sold worldwide by September 2006. Taking the number of tracks sold on iTunes and dividing it by the number of iPods, Forrester Research worked out that although the iPod may be able to hold 1,000 songs, the average person pays to download just 22.

"IPods are not sitting around generating dozens and dozens of transactions every quarter, " said Josh Bernoff, a principal analyst for Forrester. "People buy a certain number of songs, and then they stop."

The number seems surprisingly low for a service that many analysts thought was catalysing a whole new lease of life for the global music industry. It will be food for thought for those analysts this Christmas to figure out why people aren't spending to download more tracks.

It could be there are only so many times you can stand buying the White Album, no matter how good the new music format.