UPDATE - The NPD Group has begun tracking paid online subscription markets for games; digital distribution tracking may begin "later this year."

Update: Added comments from NPD.

Every month, NPD releases videogame retail sales data for the US. But there’s been something missing from the overall picture: paid online subscriptions.

NPD will now be releasing the quarterly Video Game & PC Game Subscriptions Report, which will provide data on “market size and title shares for paid online subscriptions to videogames and MMO/PC games, gaming Web sites, and videogame console services.”

Quarterly revenues will be one of the key pieces of data included with the report, NPD told Next-Gen.

Data is drawn from “thousands” of gamers 13 years of age and older who are part of NPD’s 3 million-person consumer panel.

“These products are an important step towards NPD's ultimate goal of providing a truly complete view of the North American PC game and videogame markets, including all of the different ways consumers pay for and play videogames, PC games, and related products,” said NPD Group VP Martin Zagorsek.

Paid online subscriptions have become a huge revenue generator for many game companies. For instance, in North America alone, 2.5 million gamers pay $13-$15 every month to play Blizzard’s World of Warcraft, but NPD's previous data offerings don't take those figures into account.

Millions of Xbox Live Gold subscribers also pay $50 per year for the online service.

“The service will include subscriptions to Xbox Live,” Zagoresek confirmed to Next-Gen. “And if other consoles begin offering premium (paid) subscriptions in the future we'll track those as well.”

While the new report means NPD will track subscriptions, the firm still has yet to track the growing paid downloadable games market.

“The reasons we're not tracking that yet are a technical, the methodology is a bit different,” Zagorsek said. “We're hoping to address downloads later this year.”

NPD also said it would also begin tracking subscriptions in non-game markets including security, tax, personal finance and online storage.