A new study by the Online Publishers Association has found that 61% of tablet users have purchased some type of content on their devices in the past year, whether it be a movie, book or news subscription. Of those, 39% have bought either a magazine subscription or a single issue of a magazine in the last 12 months, and 15% have bought a newspaper subscription.However, at the same time, there’s a growing preference for free, ad-supported apps: The same study found that 54% of tablet users prefer free, ad-supported apps over paid ones, up from 40% in 2011. And 19% said they would pay more for ad-free apps, down from 30% the previous year.So, as usual, subscription site execs are left scratching their heads. Of course, one should be wary of such findings as they are usually culled from B2C, generalized markets. Smart subscription site execs know they have to test what works for their individual market. And in most cases, a hybrid revenue Model of both subscription payments and advertising is best.To help with that end, Nielsen just announced that it has a measurement system to capture iPad and other tablet usage. However, the system is not yet widely deployed and may only be available to large media organizations at first. Still, a more accurate measurement of tablet circulation could help subscription sites grow both circulation and advertising revenues.