Study Information

The Oregon Health Insurance Experiment is a landmark, randomized study of the effect of expanding public health insurance on the health care use, health outcomes, financial strain, and well-being of low-income adults. This study uses a randomized controlled design – the gold standard for medical evidence – to evaluate the effects of insurance.

  • Although there are many studies comparing health or health care use between the insured and uninsured, inferring the impact of health insurance from such comparisons is difficult because the insured and the uninsured may differ in many ways – such as income, employment, or initial health – that may themselves affect the outcomes being studied. This makes it difficult to discern the effects of insurance itself.
  • Random assignment of health insurance to some but not others avoids such confounding factors (since the treatment and control groups are divided by chance, eliminating systematic differences between them). Such an experiment has never before been performed in the Medicaid program, however, in part because there would be ethical concerns with deliberately withholding available health insurance. The allocation of the limited number of spots available in Oregon’s insurance program, however, created a natural circumstance that allowed researchers to take advantage of such a randomized design.
  • The Oregon health insurance lottery provided the unique opportunity to gauge the effects of health insurance using the rigorous standards of a randomized controlled trial.
    • In early 2008, Oregon opened a waiting list for its Medicaid program for low-income adults that had previously been closed to new enrollment. Approximately 90,000 people signed up for the available 10,000 openings.
    • The state drew names from this waiting list by lottery to fill the openings. The state deemed random selection by lottery the fairest way within federal guidelines to allocate its limited number of openings.
    • This random selection allows researchers to gauge the many effects of health insurance itself, isolating it from the types of confounding factors that can plague observational studies.
  • The study compiles rich data from many sources to examine a wide range of potential effects of insurance, including both primary data from surveys and administrative data such as hospital records.

Trial Registry

The Oregon Health Insurance Experiment is registered with the American Economic Review registry for randomized controlled trials at:

Study Overview

The Abdul Latif Jameel Poverty Action Lab summarizes the study and results to date in this brief.

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