Sample e-norms plot. Cumulative density (curve) first-order difference (dots) of a simulated variable with 1,000 data points and a mean of 20 and standard deviation of 1.5. When a straight-line fit is overlaid on the inverted S curve, the plot of the first-order difference reveals the range of data points with low first-order difference, helping to identify inflection points A and B. Reprinted by permission from Wolters Kluwer Health, the Journal of Clinical Neurophysiology: Jabre JF, Pitt MC, Deeb J, Chui KKH. E-norms: a method to extrapolate reference values from a laboratory population. 2015;32(3):265–270. Copyright 2015 by the American Clinical Neurophysiology Society.