As discussed earlier part of the scatter in spirometric test results (in this case FEV1 in Caucasian females, red dots) can be explained by differences in age and height. Using the equation
log(Y) = a + b•log(height) + c•log(age) + spline
we can derive an average predicted value by substituting age and average age for height (black line), and the predicted value using actual age and height for each individual (gray dots). By eye-balling the graph you can see that up to half of the scatter remains after taking height into account. Thus, there are other factors than age and height which cause the FEV1 to differ between Caucasian females of the same age and stature. Some of it relates to within-person and between-person variability, which may be short-term, related to time of the day (circadian) or to seasonal variation, and also to other factors.