According to NPD data and analysis compiled by Gamasutra, the XBox 360's tie ratio -- a comparison of software units sold for every hardware unit -- is now 8.1 to 1, overtaking the Wii and PlayStation 3's significantly lower tie ratios. "They can be an indication of the health of a system,” says NPD Group analyst Anita Frazier, on the value of tracking tie ratios. "If a hardware system is doing gangbuster sales, then the tie ratio can go down even if there are lots of overall sales.” Gamasutra obtained the following up-to-date statistics on the lifetime tie ratio of the major next-gen consoles in the U.S., up to and including September 2008:

Referring to the lifetime-to-date tie ratios above, one can infer that each Xbox 360 owner has purchased on average 8.1 games, while Wii owners have purchased 5.5 games, and PS3 owners have purchased 5.3 games. As with any average, it is a statement about the aggregate, not the individual. For reference, at the end of 2007, the Xbox 360's tie ratio was 6.98, Wii's 4.64, and the PlayStation 3's 4.26. Our estimates put the Wii's tie ratio at 5.5 since June 2008, and we believe that it has held roughly constant since. The Xbox 360's tie ratio, however has increased by about 0.1 each month since early 2008. With these figures, one can also estimate how much software each platform has sold overall and during the first nine months of 2008: The Xbox 360, which was released a year prior to the Wii and PlayStation 3, has sold 91 million software units in its lifetime, with 27 million of those sold in 2008. The Wii has sold 69 million software units overall, with just over half of those sales from this year. Sony's console has sold only 29 million units so far, with over 15 million of that from 2008. Each company has its own positive angle that it can push based on thee figures. While Microsoft can point out that Xbox 360 owners buy more software, per system, than the owners for either of its competitors, Nintendo can emphasize that it is not only moving the most software in absolute terms, but it's monthly software unit sales are up nearly 60% this year. Sony can argue that software sales are getting much better on the PlayStation 3, with monthly averages up 73% this year.