Abstract
Medicare adjusts payments to Medicare Advantage (MA) insurers using risk scores that summarize the relationship between fee-for-service (FFS) Medicare spending and beneficiaries' demographic characteristics and documented health conditions. Research shows that MA insurers have increasingly documented conditions more thoroughly than traditional Medicare-resulting in higher payments to insurers-but little is known about what factors contribute to diverging risk scores. We apportion that divergence between market-wide increases and increases that vary with length of MA enrollment. We also examine whether effects vary across plan types and whether the enrollment duration effect is contingent upon remaining with the same insurer. Using Medicare administrative data from 2008 to 2013, we employ a difference-in-differences model to compare the growth in risk scores of Medicare beneficiaries who switch from FFS to MA to that of beneficiaries who remain in FFS. We find that the effect of MA enrollment on risk scores increased from 5% in 2009 to 8% in 2012 and that continuous enrollment in MA was associated with an additional 1.2% increase per year, regardless of continuous enrollment with an insurer. Thus, even among those who switched to MA in 2009, enrollment duration comprised less than one-third of the coding intensity difference in 2012. We also find that risk scores grew faster in areas with greater MA penetration and among Health Maintenance Organization enrollees. Overall, our findings suggest that market-wide factors contributed most to the increasing divergence between FFS and MA risk scores.