Welcome to Health Reform Watch, Sarah Kliff’s regular look at how the Affordable Care Act is changing the American health-care system — and being changed by it. You can reach Sarah with questions, comments and suggestions here. Check back every Monday, Wednesday and Friday afternoon for the latest edition or sign up here to receive it straight from your inbox. Read previous columns here.

On Jan. 1, Obamacare's huge coverage expansion officially begins. But with millions of insurers canceling health insurance plans and many Americans finding it difficult to sign up for coverage on HealthCare.gov, will the number of uninsured go down -- or up?

Health care experts say that Obamacare's Medicaid expansion is likely to offset any cancellations and drive the ranks of the uninsured down. But they acknowledge that deciphering the truth will be nearly impossible, especially when it comes to figuring out whether private insurance coverage went up or down.

There's no one data source to tell us whether, when the ball drops in Times Square, more Americans will have private health insurance.

"There’s no way we’ll be sure," says Larry Levitt, senior vice president of Kaiser Family Foundation. "There are a whole number of uncertainty and missing pieces."

"It's the exact opposite of weather forecasting," says Stan Dorn, a senior expert at the Urban Institute, who has spent decades studying health coverage expansions. "There, you can be pretty confident of what will happen tomorrow but no idea about the future. Here it's the reverse: Over time there will be significant gains, but that will take years, not months."

Much of the challenge stems from the fact there's no one data source that tracks who is gaining insurance coverage and who has lost, or canceled, their policy. The data that we do have can be difficult to piece together. Some states have estimated the number of Americans whose insurance policies were canceled under the Affordable Care Act, according to the National Association of Insurance Commissioners. They've done that by by sending out surveys to health insurers asking them what products they plan to offer in the coming year and which will be discontinued.

This is how Pennsylvania has estimated that 250,000 people had their policies canceled for 2014 and how North Dakota arrived at a figure of 36,000 plans ended. But not every state has an estimated number like these.

Each state has data on who has enrolled in coverage through Healthcare.gov or the state's own exchange, released monthly by the Obama administration. This is how North Dakota, for example, knows that 265 people have picked plans on the exchange and another 1,001 been found eligible for Medicaid.

On a first read the numbers look devastating for the Affordable Care Act. North Dakota, for instance, has a gap of more than 34,000 people who had insurance and then saw their plans canceled and haven't signed up for new coverage through Obamacare. But the data is incomplete: It doesn't tell you how many people who had their policies canceled decided to buy a new one off the exchange. Insurers don't have to tell the government when this happens.

Late Thursday, the White House did release an estimate on this front, saying that fewer than 500,000 people whose policies were canceled had not yet enrolled in coverage. Since this type of information is not typically reported to the government, some health policy experts I spoke with were confused (or even skeptical) about how it was calculated.

"I've been puzzled," says Caroline Pearson, a vice president at research firm Avalere Health. "I can't feel super confident that's a very strong estimate. It feels like an impossible number to estimate."

"I want to know the basis of it because there are, obviously, a lot of smart people at Health and Human Services," Dorn says. "Maybe they've been able to figure out a way to do this."

A White House spokesperson did not respond to a request for more information on the methodology. In a briefing with reporters Thursday, one senior administration official said, "I would say it is actually less than half a million, but we can go back and see if we have some more specific numbers."

If the 500,000 figure is accurate, most experts say, strong Medicaid enrollment nearly guarantees an increase in overall enrollment. They're less certain though, on how the private insurance market shakes out.

The Obama administration estimated Friday that 4 million people have enrolled in just the Medicaid program. That bigger bump in public coverage would almost certainly mean more people will have some type of coverage in January than they do in December.

"If it's really only a half-million, and more of those people either buy an exchange plan than choose the hardship exemption, then coverage goes up," Pearson says. She points out that these are people who already elect to buy insurance without a government mandate to do so, indicating they'd be likely to do so next year, too.

The landscape in just the private insurance market is a bit less certain, with some state officials projecting that fewer will have those plans in January than did in December.

"I think what's going to happen in my state, which has a very low uninsured rate, is you may see in the first few months a spike in the uninsured rate," North Dakota insurance commissioner Adam Hamm says.

His state has the lowest enrollment on the HealthCare.gov site, with 265 people picking plans. His insurance plans tell him that, of those sign-ups, about 185 have paid the first month's premium necessary to gain coverage. But he doesn't know how many among those who received cancellations have enrolled off the marketplace.

Hamm believes the real test won't be January but rather the end of March, when open enrollment ends.

"Right now its like we're watching the preview before the real movie," he says. "And the real movie is going to be the answers to two fundamental questions: How many people enroll by the end of March? And what's the actual risk pool?"

Without that, he says, its impossible to declare an Obamacare failure or a success.

KLIFF NOTES: Top health policy reads from around the Web.

Some industry groups are giving up on the Medicaid expansion fight. "Support for Medicaid expansion from within the health care industry -- in particular from providers like hospitals, doctors and nursing homes -- is a key part of the coalition that Obamacare advocates have hoped will help eventually turn the tide in the non-expanding Republican-dominated states. But top officials for powerful trade organizations in three of the largest states not expanding Medicaid under Obamacare told TPM that they have effectively given up that fight until political conditions change, setting their sights on 2015 at the earliest." Dylan Scott in Talking Points Memo.

Insurers pushing for last-minute enrollment are finding it to be a tough sell. "Insurers pressing for last-minute enrollees under the health-care law say they are running into a worrisome trend: Customers who were put off by the insurance marketplaces' early troubles are proving hard sells. Many people thwarted by the technical problems of HealthCare.gov are reluctant to try again, citing frustration with the federal site, web-security concerns and the pressure of the holidays, several insurers say." Timothy W. Martin and Christopher Weaver in the Wall Street Journal.