Fallacy of Per-Weight and Per-Surface Area Standards, and Their Relation to Spurious Correlation

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Abstract
The present standards which express physiol. functions such as O2 consumption and cardiac output as per-weight or per-surface area are theoretically fallacious, and in practice (except in a special case) misleading. Their use in cardiac output.studies results in large men being too easily considered to have relatively small outputs, and small men relatively large ones. The ratio standard does not lead to the same result as a regression standard The theoretical aspect of the fallacy is discussed, and it is shown to be a special example of the spurious correlation of indices. A number of practical examples where the fallacy has misled investigators are given, concerning cardiac output, O2 consumption in child and adult, plasma volume renal blood flow and filtration rate. The assumption that the best ratio standard is the one which minimizes the spread of the data is also criticized. Finally the conditions are examined under which the power standard (e.g. O2 = wt[alpha]), as usually obtained, is valid; and the more common conditions where it is invalid unless a is obtained by Sholl''s method.

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