I am certain that I have made this mistake myself, but Kevin Drum is careless about using data just because it 1) is labeled in a way he thinks he understands and 2) it supports his pre-conceived notions.
He tries to use the above chart to make the point that Medicare is superior to private insurers because it is more "accurate." Accuracy in claims seems like a good thing, but I started to wonder how it was defined in this study.
So I spent like 30 whole seconds clicking through to the study. It turns out the data is based on surveys of doctors. This chart is explained this way:
Description: On what percentage of claim lines does the payer's allowed amount equal the physician practice's expected allowed amount?
So really, this chart is not a measure of insurance company accuracy, it is really a measure of doctor accuracy in estimating insurance company claims payment behavior, or perhaps of insurance company claims transparency. Because Medicare pays fixed, published, below-market rates, and because they are so large, it is not at all surprising doctors are better at predicting what Medicare will pay on a claim.
In other words, doctors disagree with Aetna on claims more frequently than they disagree with Medicare? Is this bad or good. I have no idea.
But one could go further and say that another way of heading this chart, rather than "accuracy," would be "willingness of insurer to roll over and pay whatever the doctor asks for."
In the past, Drum and others on the Left have also bragged that Medicare's overhead is lower than private insurers. These are all related issues. Private insurers put more scrutiny on claims, which costs more in overhead and causes claims to get paid slower, but presumably results in lower claims payments and less fraud.
Medicare's approach may be net better (ie overhead savings could be larger than claims and fraud savings) or it could be worse, but this chart in isolation tells us nothing.