Female physicians at some of the nation’s most prominent public medical schools earn nearly $20,000 less a year on average than their male colleagues, according to an analysis published on Monday in JAMA Internal Medicine.

Before adjusting for factors that could influence income, the researchers found that the absolute difference between the genders was more than $51,000 a year.

Several studies have found a persistent pay gap between male and female doctors. But those reports relied mostly on doctors reporting their own incomes, or focused on pay disparities in one specialty or one region, or on starting salaries.

The new study draws on salary information from a much larger, objective sample. The researchers went to great lengths to account for a variety of factors that can influence income, such as the volume of patients seen by a physician and the number of publications he or she had written.