The largest occupations in the U.S. last year were mostly low-paying jobs like salesmen and waitresses, a new report shows.

The 10 largest fields accounted for about 20% of the workforce last year — and they tended to include some of the worst-paying positions, the Labor Department said Tuesday. Heavily concentrated employment in low-paying jobs has been a concern for some economists, who note the economy is creating more low-paying jobs and more high-paying jobs (that require higher education) and leaving fewer opportunities for the traditional American middle class.

Retail salespersons and cashiers were the largest fields last year, employing 7.6 million — 6% of total employment. Also included were food preparation and serving workers, cashiers, and waiters and waitresses — the lowest paying of the largest fields and among the poorest-paying jobs of the nearly 800 occupations the Labor Department tracks.