Inertia in physicians practice patterns

There is wide variation in medical practice patterns between regions, hospitals, and physicians. Evidence from clinical trials may reduce unwarranted practice variation, but only if physicians modify their beliefs accordingly. “Cognitive inertia” – the persistence of beliefs in the face of contradictory evidence – may limit the impact of evidence on medical practice.

We evaluate the degree to which physicians’ use of episiotomy is subject to inertia. Episiotomy is a surgical procedure to enlarge the vaginal opening during childbirth. Randomized trials published from 1984 onward have found that routine use of episiotomy does not benefit mothers or babies.

The choice of whether or not to perform an episiotomy is a good scenario in which to study inertia in physician decision making. Obstetricians do not receive extra payment for performing episiotomies, and so practice patterns ought to respond to changes in beliefs. There are no switching costs or technological lock-in effects that prevent obstetricians from reducing their use of episiotomy.

We analyze episiotomy rates using a 100% sample of hospital discharges in Pennsylvania over the period 1994 to 2010. The data include physician identifiers linkable across years, allowing us to track physician-specific episiotomy rates over time. The use of episiotomy decreased from 42% of non-operative vaginal deliveries in 1994 to 12% in 2010. In any given year there are approximately 1,000 physicians who deliver babies. There are between 50 and 100 physicians who enter practice each year (and a similar number who exit).

We test for inertia by examining the impact of physician tenure on the use of episiotomy. We find that older, more experienced physicians reduced their use of episiotomy over time, but performed episiotomy at much higher rates than younger physicians. These differences are apparent even as late as 2010. For example, in 2000 there was a 7 percentage point difference (30% versus 23%) in episiotomy rates between physicians who entered in 1994 or earlier compared to physicians who entered in 1995-1998. One decade later, in 2010, there was still a 3 percentage point difference (13% versus 10%). The rate among physicians who entered 2009-2010 was only 5%.

We estimate a structural model that exploits the fact that physicians who are new to practice in each year are not subject to inertia. Results indicate that in the absence of inertia, episiotomy rates in 2010 would be 2 percentage points lower (6.1% versus 7.9%) (p < 0.01).

We find that there is a clinically-significant degree of inertia in physicians’ practice patterns in this context. Results are useful for understanding variation in practice patterns and suggest caution in permitting technologies to diffuse into routine practice before they have been evaluated.