Figure 1.
Residual plots (actual minus mean linear regression predicted brain volumes by age) from the normal (top panel; n=227) and AD (bottom panel; n=219) samples.
There are skewed residuals at 70 years in the normal sample (top) and a similar pattern at 85-89 years in the AD sample (bottom). This means that the linearity assumption was in question.
Figure 2.
Mean (top panel) and percentile rank (bottom panel) regression estimates of brain tissue volume across age in the normal sample (n=227).
The slopes of these lines represent the beta coefficients in table 4. The mean-based model expects all percentile ranks to change at the same rate, i.e. be parallel. The diverging percentile ranks show that this is not the case and that variance in brain volume generally increased with age in the normal subjects. Although some may appear linear, each line is the result of nonlinear regression (table 4).
Figure 3.
Illustration of the varying differences in normal ageing brain tissue volume, according to percentile rank.
There were much greater differences between ages at the 5th percentile of brain tissue volume (bottom panel) than between ages at the 95th percentile (top panel) of normal subjects.
Figure 4.
Mean (top panel) and percentile rank (bottom panel) regression estimates of brain tissue volume across age in the AD sample (n=219).
The slopes of these lines represent the beta coefficients in table 5. The mean-based model expects all percentile ranks to change at the same rate, i.e. be parallel. The converging percentile ranks show that this is not the case and variance in brain volume generally decreased with age in the AD subjects. Although some may appear linear, each line is the result of nonlinear regression (table 5).