The Gallup organization is out with yet another poll, looking at how many Americans have health insurance. And once again, the percentage without coverage has fallen. According to the survey, the percentage of uninsured Americans is down to 15.9 percent. That’s lower than its peak last year and lower than it was in March 2010, when the Affordable Care Act became law.

So does this prove Obamacare is working? No. It's a sign of progress, but only a sign. Gallup is probably the best available source for real-time data on the uninsured rate. And the pattern Gallup detected—unusually large increases in coverage among African-Americans and Latinos—would be consistent with a program that benefits low-income groups the most. But Gallup's survey is not as reliable as the big government surveys on the uninsured, which won't be available until next year. In addition, the Gallup data for last year, 2013, shows a very strange pattern, with the uninsured rate spiking to 18 percent in the middle of the year for no apparent reason. That makes it hard to be certain exactly what's happening right now.

As Greg Sargent says, "caution is in order." Larry Levitt, senior vice president at the Kaiser Family Foundation, summed it up this way:

Gallup’s numbers are volatile and need to be interpreted cautiously. But, at the same time, there’s now mounting evidence with Medicaid and exchange signups that the number of Americans who are uninsured is dropping as the ACA has gone into effect.

Of course, not everybody reacts to news about the health care law so carefully. Over the last few weeks, Obamacare critics have made a great deal of noise about surveys that put Obamacare in a poor light—including a study from McKinsey showing that only a small portion of people getting insurance through the exchanges lacked coverage before. (Here's a dispatch from Fox News, just to take one example.)