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Should we be using statistics to define disease?
  1. David M Mannino
  1. David M Mannino, Department of Preventive Medicine and Environmental Health, University of Kentucky College of Public Health, 121 Washington Avenue, Lexington, KY 40536, USA; dmannino{at}

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“Don’t worry about the physiology test—if everybody fails, everybody passes”, Anonymous, Jefferson Medical College, Fall 1977.

The first few months of medical school caused anxiety in most students, who typically went from environments where they were well above the “norm” in their classes to one in which they were just average. At the same time, the volume and difficulty of the material one was expected to master increased dramatically, compared with undergraduate studies. The one hope students could hold onto was that tests were graded “on the curve” and as long as you did better than the bottom 5% of the class (the lower limit of normal (LLN)) you would pass. This is the origin of the phrase “if everyone fails, everyone passes”. At the time, unbeknownst to the students, there was a push in our medical school to move towards a more standardised minimal passing grade of, somewhat ironic to this discussion, 70%, that would allow better comparison between classes and schools. The argument here was that if, in fact, everyone failed (ie, scored less than the minimum passing grade), even if that was “normal”, it was not good.

The conflict between what is statistically “normal” and statistically “abnormal”, and how these are defined, is central to a current controversy in the world of respiratory medicine. On one side of this debate is the idea that “normal” people lose lung function as they age, and because of the “normal” loss of elastic tissue in the lung, the forced expiratory volume in 1 s/forced vital capacity (FEV1/FVC) will also decrease with aging. Defenders of this position state that the definition of “abnormal” needs to …

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