Health insurance premiums are lower in state-based health insurance marketplaces and areas with denser populations, according to a new study from the Brown School at Washington University in St. Louis and the University of Iowa College of Public Health.

Researchers analyzed premiums in 500 rating areas across the United States in 2014, with health insurance marketplaces created under the Affordable Care Act. They adjusted the premiums to account for other factors, such as the different values of the plans and cost-of-living differences.

They found that average adjusted premiums in state-based marketplaces were about $20 lower than premiums in states whose marketplaces were federally operated. Adjusted premiums were higher in the Midwest, and in areas with less-dense populations and fewer health care providers.