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Scatter in spirometric indices is not proportional to predicted value

Scatter around FEV1 is independent of FEV1 level

As shown previously the scatter about the predicted value does not vary proportionately with standing height and age. Yet, the older and shorter the subject, the lower the FEV1 and FVC. A subject of 1.80 m and 25 yr whose FEV1 is three standard deviations below the predicted value (z-score = -3) therefore has an FEV1 which is comparable to that of a 70 yr old subject of 1.80 m in whom the FEV1 is similarly 3·RSD below the predicted value. If we were to express the observed value as a percentage of the predicted FEV1, it would seem that the 70-yr old man was in a much poorer shape: 53% predicted, in the younger person 67% predicted. This artifact is due to inadvertently dividing one figure by the other even though they are not proportionally related. If the 70-yr old man were only 1.65 m tall the apparent discrepancy between the two men would be spuriously increased further.
Even though the z-score carries the same statistical message at any age, this does not necessarily apply to the biological message. A low FEV1 is often due to an abnormally rapid decline, a prognostically unfavorable sign; therefore, in the present example, at age 70 yr the younger of the two men may be in a poorer position than his elderly counterpart.

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