Heavy and tractor-trailer truck driver was the hardest occupation to fill in the US, according to the American Staffing Association’s second-quarter skills gap index released today, a quarterly index that tracks hard-to-fill occupations in the US.

The index identified 158 occupations as hard to fill for the second quarter of 2016. The top 10 are:

1. Heavy and tractor-trailer truck drivers
2. General internists
3. Podiatrists
4. Psychiatrists
5. Physician assistants
6. Merchandise displayers and window trimmers
7. Obstetricians and gynecologists
8. Surgeons
9. Occupational therapists
10. Nurse practitioners

Physician assistants, obstetricians and gynecologists, and nurse practitioners moved into the top 10 list of hardest-to-fill positions. Healthcare continues to dominate the list with eight of the 10 most difficult-to-fill occupations falling within that sector. Occupational therapist assistants, physical therapists and family and general practitioners slipped below the top 10 because of methodology changes.

“More than 15 million people in the US are unemployed, including those marginally attached to the labor force, or are working part time for economic reasons, yet there are still nearly six million job openings going unfilled,” said ASA President and CEO Richard Wahlquist. “Staffing, recruiting, and workforce solutions companies are helping businesses strategically recruit and retain talent by using the ASA Skills Gap Index to ensure they are paying wages that are competitive based on regional variations and current skills shortages.”

The ASA Skills Gap Index draws on 90-plus data sources and utilizes CareerBuilder’s Supply & Demand Portal Hiring Indicator scores.

In September, CareerBuilder implemented new guidelines for its supply and demand data methodology and salary data inputs, significantly changing hiring indicator scores and the number of hard-to-fill occupations.