It can be hard to measure something that keeps growing.

Just over two weeks ago, the Pew Research Center’s Project for Excellence in Journalism reported that there are roughly 150 dailies in the United States that now have some form of digital subscription service. “Dozens more papers are likely to follow in 2012,” the study’s authors wrote.

Fast forward to Wednesday, and here’s the headline attached to an AP story in the Washington Post: “Nearly 300 US newspapers now charging for access on websites, smartphones and tablets.”

I had heard about the Lansing State Journal (Go Green!) announcing its paywall the other day — but did the number of dailies with paywalls really double in a matter of weeks?

It depends on who you ask and how you count.

Most of the papers AP is counting are clients of Press+, which announced in a press release that 323 publications now use its services to “launch paid models.” But those aren’t all newspapers, or in the United States.

“Press+ also does college publications, magazines, trade pubs, things like that,” said Chuck Moozakis, editor-in-chief at News & Tech, which produced the numbers Pew cited. Press+ also works with non-daily newspapers.

Press+ spokeswoman Cindy Rosenthal says that, if you take out the weeklies, magazines, international newspapers, and others, the right number for their platform is around 250 — that’s U.S. dailies who use the company’s platform for digital subscriptions. She couldn’t give an exact figure, and Press+ keeps confidential its list of clients, which makes comparing their count to others’ difficult. But she noted the number continues to grow.

By Moozakis’ count — he updated it just a few days ago — the number of U.S. dailies with digital subscription services is hovering around 160. And the Newspaper Association of America‘s most recent data (from mid-February) finds about 110 daily newspapers in the U.S. have paywalls.

One thing seems clear: The number of newspapers that are putting up paywalls is on the rise. Between newspapers owned by Lee Enterprises Inc., and Gannett, Moozakis estimates about 110 new paywalls in the nearish future.

“Other publishers like McClatchy, which at this point only has one paper with a paywall, you gotta figure they’re going to be jumping on this,” he said. Tribune is the “wild card.” Even the company’s home page has dueling perspectives under the subheads “Old Media is Dead” and “Long Live Old Media.”

Turns out the lesson here is one we keep learning again and again: The news business is changing, and it’s changing fast.