One of the chief goals of the Affordable Care Act was to expand insurance coverage so that all Americans could have access to quality health care. How's that working out?

According to the Department of Health and Human Services, about 20 million people have gained coverage because of the ACA — either signing up for insurance through one of the marketplaces established by the law or enrolling in Medicaid or the Children's Health Insurance Program.

All told, the sign-ups drove the uninsured rate down from 15.7 percent in 2011 to 10.1 percent in 2014, according to the latest figures from the U.S. Census Bureau. A further decline in 2015 is anticipated, given the increased volume of people signing up for insurance last year.

But behind these positive trends lie areas of deep concern to health policy leaders. In a series of polls, NPR, the Robert Wood Johnson Foundation and the Harvard T.H. Chan School of Public Health found that a surprising share of Americans, particularly those with low incomes, are having problems with the quality of care they receive.

We found that among people living in households with incomes of less than $25,000, 34 percent rate the quality of health care they receive as fair or poor. That compares with just 13 percent of people with higher incomes.

The significance of this finding is that low-income people are more likely to be sick than people of other incomes. Forty-one percent of low-income adults rate their health as fair or poor, versus just 18 percent of people of other incomes.

The polls are part of an in-depth study to assess the changing health care landscape in the two years since the ACA took full effect. We conducted a national survey of 1,002 people, along with seven separate state polls of approximately 1,000 people each in Florida, Kansas, New Jersey, Ohio, Oregon, Texas and Wisconsin.

Other findings show the use of emergency rooms has not declined as many had predicted prior to the ACA. The use of urgent care centers, which are often lower in cost, is rising.

As part of our series, "Perceptions of Health," Harvard is presenting a webcast Wednesday at 12:30 p.m. ET in collaboration with NPR and the Robert Wood Johnson Foundation to discuss whether efforts to extend coverage have improved the health care experiences of Americans and narrowed the gap in access to high-quality care between low-income and other Americans.

Joe Neel, deputy senior supervising editor on NPR's Science Desk, will moderate a discussion with: