Evidence About “Customary Practice”

by Christopher Bruce

This article was originally published in the Spring 2001 issue of the Expert Witness.

The standard of care that is expected of a commercial enterprise is often determined by examining the “customary practice” followed by businesses in the defendant’s industry. Obstetricians are compared with other obstetricians; taxi drivers with other taxi drivers; and police departments with other police departments.

The determination of what constitutes the customary practice in an industry is usually left to the testimony of experts drawn from that industry.

In a recent University of Chicago Law School working paper, William Meadow (Associate Professor of Pediatrics) and Cass Sunstein (Professor of Jurisprudence) warn that such expert testimony is likely to be systematically biased. (“Statistics, Not Experts,” John M. Olin Law & Economics Working Paper No. 109, (2d Series) 2000.)

Their argument is not the traditional one – that experts will be reluctant to testify against their colleagues and, therefore, may understate the level of precautions “normally” taken. Rather, they argue that experts will systematically overstate the level of precautions that are normally taken, thereby raising the implicit standard against which defendants will be measured.

M & S base their argument on the oft-noted observation that “most normal people tend to be risk optimists, in the sense that they believe themselves to be relatively immune from risks that are faced by similarly situated others.” For example, they report that 90 percent of drivers believe themselves to be less likely than the average to be involved in a serious accident; and most heavy smokers believe they are not at increased risk of cancer or cardiovascular disease.

M & S report that physicians have been found to be particularly susceptible to this “optimism bias.” In one study, for example, 88 percent of doctors overestimated length of survival for seriously ill patients, by roughly a factor of three. In another, doctors made inaccurate predictions in 80 percent of cases, with overestimates in 63 percent. In a third study, physicians accurately predicted the survival time of cancer patients in only 10 to 30 percent of cases, and the rest of the time they overestimated survival by a factor of two to five.

M & S predict that this proclivity to optimism will affect doctors’ (and other experts’) ability to provide correct estimates of “customary practice.” In particular, they predict that this optimism will lead doctors to overestimate the ease with which they and their colleagues can recognise and treat symptoms and to underestimate the time required to react to medical emergencies.

To test this hypothesis, M & S asked a large number of emergency room physicians to estimate the average time that would elapse between the arrival of a child with bacterial meningitis in their emergency room to the start of antibiotic therapy for that child. They contrasted these estimates with statistics of actual times elapsed that they were able to obtain from their own study of two Chicago area medical centres and from two studies reported in the academic literature.

What they found was that the actual elapsed times were almost double the estimated times. Whereas the physicians’ average estimate was 65 minutes, the statistical studies revealed an actual average of 120 minutes.

Imagine now that a hospital has been sued in negligence for failing to treat a child within a “reasonable” time. If that hospital had treated the child within 110 minutes, it would actually have outperformed the average. But the average “expert” witness would have testified that most hospitals would have treated the child within 65 minutes. The behaviour of the defendant would be found to have fallen below the standard of “ordinary practice.”

(Of course, this does not necessarily mean that the defendant would be found negligent, as the court could conclude that the average hospital took less time to treat children than was required. However, this would be an unusual outcome.)

M & S further argue that this overestimation of the standard of ordinary practice will be common not only to physicians but also to experts within most other disciplines – from engineers to truck drivers – because they believe that most experts share doctors’ optimism. The result is that use of such experts will systematically bias the finding of negligence in favour of the plaintiff.

They conclude that:

  1. The courts should be very skeptical of testimony that attempts to identify ordinary practice based solely on the estimates of “expert practitioners.”
  2. Wherever possible, statistical evidence should be used in preference to practitioner evidence when determining ordinary practice.


Christopher Bruce is the President of Economica and a Professor of Economics at the University of Calgary. He is also the author of Assessment of Personal Injury Damages (Butterworths, 2004).