That is the new paper by Christine S. Spencer, Darrell J. Gaskin, and Eric T. Roberts. Let me excerpt the three most important sentences from the abstract:
We found that privately insured patients had lower risk-adjusted mortality rates than did Medicare enrollees for twelve out of fifteen quality measures examined. To a lesser extent, privately insured patients also had lower risk-adjusted mortality rates than those in other payer groups. Medicare patients appeared particularly vulnerable to receiving inferior care.
I don’t have a great deal of confidence in our ability to estimate the size of that effect, but keep that difference in mind next time someone tells you that Medicare is so much more efficient than private health insurance in this country.