What is the effect of expanding Medicaid on overall healthcare costs and use of the emergency room?
This type of question can't easily be answered by observational studies and requires a randomized controlled trial (RCT). But an RCT isn't easy to perform. However, a natural RCT serendipitously happened in Oregon a few years ago when Medicaid was expanded and the eligibility was deemed by a lottery system.

Saurabh Jha (@RogueRad) speaks to Professor Katherine Baicker, a leading economist and the Dean of the Harris School of Public Policy, and principal investigator of the Oregon Health Insurance Experiment, about the landmark study.

Links:

The Oregon Experiment — Effects of Medicaid on Clinical Outcomes

Medicaid Increases Emergency-Department Use: Evidence from Oregon's Health Insurance Experiment

Effect of Medicaid Coverage on ED Use — Further Evidence from Oregon’s Experiment