How many uninsured Americans are there, really?

Megan McArdle tries to get a handle on just how many people really were uninsured.  Is it more or less than the number of people who were insured before the PPACA hit them like a truck?

A third possibility is that we don’t have the uninsured problem we thought we had.  Most of the estimates we have for the uninsured population are really pretty crude.  For one thing, we tend to treat the U.S.'s roughly 48 million uninsured as if they were part of a discrete group, like Mormons or people who know how to play the tuba.  But in fact, people change insurance status all the time.  If you look at data from the Medical Expenditure Panel Survey, you’ll see that a lot of people are uninsured for at least a month, but if you look at who is uninsured for as long as two years, that number falls by two-thirds.  If you extend the reference period out to four years, just 7.6 percent of the population counts as “uninsured.”  That is not a negligible number, but it is less than half of the 48 million we think of as uninsured.  And it’s heavily skewed toward immigrants and the young. . . .