# Scatter in spirometric indices is not proportional to predicted value

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.