MEGAN MCARDLE wants proponents of health-care reform to commit to a series of testable propositions about the benefits likely to ensue now that the bill is (almost) law. Most prominently, she would like proponents to back up their earlier claims that lack of health insurance causes "tens of thousands" of deaths per year by committing to the proposition that the death rate will now fall by a commensurate amount. That seems like a reasonable request. Ezra Klein says he's willing to take that bet:



Why is it so hard to gather good data on this stuff? Can't we just measure the number of deaths per year and see whether it goes down? Well, as Ms McArdle recognises, it's not that simple. Even a reduction of tens of thousands of deaths per year would likely get lost in the statistical noise; in 2006, according to the CDC, there were 2,426,264 deaths in America. That was down from 2,448,017 in 2005. Those annual numbers bounce around enough that it would be a long time before you could detect a consistent trend that shifted things by a few tens of thousands a year. The "death rate", or number of deaths per population of 1,000, is similarly all over the place; it's in the neighbourhood of 8.3 at the moment, but tens of thousands of deaths per year won't shift that much.

Or let's look at this another way. In the 1980s, thousands of Americans began dying every year from a new disease: AIDS. Here's a graph of the rise, leveling off, and then drop in AIDS deaths in the 1980s and 1990s, from the CDC (annual deaths in blue).

By the early 1990s, over 10,000 people per year were dying of AIDS in America. But if you looked at the overall national death rate, you'd never even know it happened:

It will ultimately be possible to tell how many early fatalities were prevented by the addition of 30m people to the rolls of the insured by 2019. But that'll have to be measured the same way the initial estimates of early fatalities caused by lack of insurance were measured, and that will take time.

* In a follow-up post, Ms McArdle says that a reduction of some 30,000 fatalities in a given year should be noticeable, as it would be about 5% of the background rate. It took me a minute to figure out that she was referring to the background rate of fatalities in the 18-64 age group, which is the group mostly affected by the new law. This is a good point; seniors 65 and over are already covered by Medicare. And obviously the number of deaths among those under 64 is far lower than those for the population as a whole. CDC counts 10-year age cohorts starting at the fives, but in 2006, for example, there were 585,541 deaths in the 25-64 age range. So a big reduction in fatalities would be a lot more noticeable within that age range than in the population at large. It may also take a while for the fatality-reducing effect of having insurance to kick in for the individuals who get coverage, but Ms McArdle argues the horizon should be no longer than a decade or so based on the methodology used in the studies that initially showed tens of thousands of early fatalities due to lack of coverage.

A remaining difficulty might be distinguishing a drop in fatalities due to insurance coverage from the drop in fatalities that might have occurred anyway. While mortality in the 25-54 cohorts has been bouncing around some, mortality in the 55-64 cohort (which had the most deaths, for obvious reasons) fell steadily between 1999 and 2006, from 1022 to 891 per 100,000. A steady decrease in mortality, mostly in older cohorts, as might be expected if expanded health insurance coverage were reducing mortality, might be hard to disentangle from decreases in mortality due to improvements in care.