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You can’t control when health emergencies occur, but if you had to go to the hospital, you’d probably be better off avoiding the summer months.

At least that’s been the conventional wisdom among doctors, who know that the most experienced medical residents graduate and leave hospitals in July, just as newly minted M.D.s (i.e., last year’s medical students) arrive to start caring for their first patients. Now a new study confirms the trend, taking the first comprehensive look at death rates and complications occurring in hospitals throughout the year.

Reporting in the Annals of Internal Medicine, Dr. John Young of the University of California, San Francisco, and his team show that at teaching hospitals responsible for training new doctors, patient death rates increase while efficiency in patient care decreases during the month of July. In these hospitals, admitted patients serve as case studies used to educate future physicians on the best way to provide care; medical residents spend anywhere from three to six years as doctors-in-training, shadowing more experienced physicians as they learn how to diagnose and treat patients.

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Come July, the most experienced residents graduate, leaving behind those who haven’t logged as many hours in the clinic or in patient wards. The older residents’ departure also coincides with the entry of a new class of freshman residents — new doctors who are taking on the responsibility of patient care for the first time.

Not surprisingly, the changeover can disrupt patient care in hospitals, increasing complications from surgery and boosting medical error rates, particularly as new doctors who are unfamiliar with a hospital’s pharmacy system mistakenly prescribe wrong doses of medications. The shift also decreases efficiency, with more unneeded or duplicate tests being ordered and patients being kept in the hospital longer than necessary.

Young’s study, which reviewed data from 39 previous studies that tracked health outcomes such as death and complications from medical procedures, found that death rates increased between 8% and 34% in July. That may be a wide range, but it’s the result of the first study to focus specifically on better-quality trials; the studies included in Young’s analysis controlled for other factors that may affect health outcomes, such as how sick patient populations were overall at the beginning of the studies.

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Each year in the U.S. the so-called July effect impacts about 100,000 staff in teaching hospitals. Young notes that such a dramatic shift in personnel rarely occurs in other industries on such a regular basis. “For me, the metaphor I think of is the football team in a high-stakes game. In the middle of the final drive, the coach sends for four new players to substitute for veteran ones. These new players have never played in the pros before, and the remaining players who do have some experience are sent to assume different positions. And the new team has never practiced together before — this is what happens every July in teaching hospitals with the physician staff,” Young says.

As precarious as that may sound — especially for patients who are being admitted into hospitals this month — hospital administrators are well aware of the July effect and have been taking steps to combat it. Some hospitals ensure that their most experienced physicians are on-call during the summer months, ready to step in and advise or supervise colleagues who might be less confident in their caregiving skills. Other centers conduct in-depth orientation sessions to make sure all new doctors are trained in the proper prescribing and caregiving procedures.

For those hospitals that don’t have such programs in place, the study shows how important instituting them can be. Patients who get sick in July shouldn’t have to get worse care than that given to patients who fall ill during the rest of the year.

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Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.