Back when the Affordable Care Act (ACA) was first being debated, there were two government estimates that predicted different numbers of new enrollees in Medicaid – one by the Congressional Budget Office (CBO) and the other by the actuaries at CMS. Since then, a number of other studies have examined the impact of the ACA on Medicaid. Well, we can add one more to the list, and one that attempts to provide some clarity on why the estimates may differ.

The most recent study, released last week by researchers at the Harvard School of Public Health and published in Health Affairs, estimates that 13.4 million people will enroll in Medicaid in 2019. However, it is important to note that by changing a few assumptions the results vary widely from 8.5 million to 22.4 million. To get to the policy piece most of note, I’m going to skim over the details on the data source differences (depending on which they chose, enrollment varied by 3.9 million people) and the income counting definitions (estimates vary by 3.1 million depending on whether income from the survey family or the health insurance unit is used).

The truly interesting piece for me is increasing the participation rate from the baseline assumption of 62% to 80% adds another 2.3 million people to Medicaid.

Now we’ve seen such success with children already. As our friend, Jenny Kenney, has shown, just 6 states have Medicaid/CHIP participation rates of less than 80% (and 16 are at or above 90%!). And Massachusetts has seen an adult take-up rate of 80% under their health reform efforts. So these higher rates are certainly achievable, they just might take a little work.

Now, policy makers (especially those trying to add up the costs and benefits) are looking for certainty in these figures. As this piece shows, we won’t likely know how many people will enroll in Medicaid until they actually do. Because as the authors conclude, “in the end, Medicaid enrollment will be determined largely by the extent to which federal and state efforts encourage or discourage eligible people from enrolling.”