Take, for example, nurse practitioners. Their average salaries are less than half that of physicians. They’re trained to provide direct primary care, and numerous studies have shown that they can deliver high-quality medicine.

There’s just one small problem: Adding nurse practitioners to a doctor’s office, it turns out, sometimes is less cost-effective than adding a new physician. That’s the surprising finding from health-care economists Nan Liu and Thomas D’Aunno in a new article in the Health Research Service.

How, exactly, does adding a lower-paid employee make a doctor’s office less productive? A lot of it has to do with the limits of what a nurse practitioner can do. State laws often require a physician to supervise a nurse practitioner, leaving that doctor with less time to see patients.

Nurse practitioners could also increase the volume of medicine provided: If a patient comes to a nurse practitioner with a more complex condition, for example, they will likely need a second visit with the physician. If they’d seen a doctor in the first place, their condition would have likely been treated in one appointment.

There are certain ways to make the hiring of nurse practitioners more cost-effective. When Liu and D’Aunno run the numbers, they find that it works if a doctor delegates at least 30 percent of his workload to a nurse practitioner, spending their newfound time performing tasks that only physicians can take on. It’s not just about hiring on alternative primary care providers, they conclude, but ensuring that their services get used in the most effective ways.