Personal experience

Although personal experience is an invaluable part of becoming a competent physician, the pitfalls of relying too heavily on personal experience have been widely documented.14-16 These include:

• overemphasis on vivid, anecdotal occurrences and underemphasis on statistically significant strong evidence

• bias in recognising, remembering and recalling evidence that supports pre-existing knowledge structures (for example ideas about disease aetiology and pathogenesis) and parallel failure to recognise, remember and recall evidence that is more valid but does not fit pre-existing knowledge or beliefs

• failure to characterise population data accurately because of ignorance of statistical principles - including sample size, sample selection bias and regression to the mean

• inability to detect and distinguish statistical association and causality

• persistence of beliefs despite overwhelming contrary evidence.17

Nisbett and Ross17 provide examples of these pitfalls from controlled clinical research, and simple clinical examples abound. Physicians may remember patients assuming that they who did not return for follow up improved, and conveniently forget the patients who did not improve. A patient treated with a given medication may develop a severe life-threatening reaction. On the basis of this single undesirable experience, the physician may avoid using that medication for many future patients, even though on average, it may be more efficacious and less toxic than the alternative treatments that the physician chooses. Few physicians keep adequate, easily retrievable records to codify results of treatments with a particular agent or of a particular disease; and even fewer actually carry out analyses. Few physicians make provisions for tracking those patients who are lost to follow up. Thus, statements made about a physician's "clinical experience" may be biased. Finally, for many conditions, a single physician sees far too few patients to draw reasonably firm conclusions about the response to treatments. For example, suppose a physician who treated 20 patients with lichen planus with tretinoin found that 12 (60%) had an excellent response. The confidence interval for this response rate (i.e. the true response rate for this treatment in the larger population from which this physician's sample was obtained) ranges from 36% to 81%. Thus, the true response rate might well be substantially less (or more) than the physician concludes from personal experience.10,18

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