The Accuracy of Physicians' Quantitative Estimates

Doctors often describe sizes by comparison with everyday objects, e.g., a pinhead-sized tympanic defect or a dehiscence the size of a penny. But do they really know how big a pinhead is? We used an internet-based questionnaire to study whether quantities are accurately estimated and whether comparisons with everyday objects improve accuracy.

In a prospective, single-center study conducted by internet-based questionnaire, physicians estimated the size of everyday objects, such as a pea or a one-euro coin, and SI units as they appeared on a computer screen and then estimated their own accuracy of estimation.

On average, the sizes of everyday objects and SI units were underestimated by 15% (95% confidence interval, [-17; -13]). The physicians' self-assessment was not correlated with their actual degree of accuracy. Board-certified specialists considered themselves better estimators than others; however, no difference in accuracy was found between specialists and resident physicians. Nor did the particular specialty have any effect on the accuracy of estimation, even though the participating radiologists and neurosurgeons considered themselves especially good estimators. The frequent use of aids such as rulers in clinical practice was not associated with a better accuracy of estimation.

Underestimates of size, such as were frequently observed in this study, can cause inaccurate descriptions and faulty decision-making in clinical practice. We therefore recommend that quantities should be measured with the appropriate instruments, and that physicians should refrain from making eyeball estimates wherever possible, regardless of their medical specialty or degree of clinical experience.