The AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Fifth Edition, changes the definition of “normal” (ie, the process of differentiating between an individual whose lung function is “normal” as opposed to an individual with Class 2 respiratory impairment) because the definition has changed over time. For example, the AMA Guides, First Edition (1971), used from the VA-Army 1961 Cooperative Study to construct tables of “normal” or “predicted” values during spirometry. Regression equations were used to calculate the predicted forced vital capacity, forced expiratory volume in the first second, and mandatory minute ventilation for men and women, by age and height. The Second Edition (1984) used data from a pulmonary function study in 251 healthy white individuals who lived 1400 meters above sea level (Utah), more than 90% of whom were members of the Mormon church (a very narrow segment of the American population). The AMA Guides, Third and Fourth Editions, continued to rely on the study just cited and made a distinction between “normal” and “mildly impaired.” The AMA Guides, Fifth Edition, uses the four classes of respiratory impairment and the same whole person impairment ratings for each class, unchanged from the Fourth Edition. The Fifth Edition has reverted to using the 95% confidence interval to determine “normal,” so that the same individual who, under the Fourth Edition guidelines was up to 25% impaired, would become normal under the pulmonary impairment guides of the Fifth Edition.