In section 2.3.1, it was observed that the trajectory, s(t), represented the cumulative effect of the developmental process, and that the state of the process at a particular age was the rate of change (time derivative) of s(t). The trajectories were presented as log(s(t)) in the top row of Figure 1. Since the derivative of log(s(t)) is (ds(t)/dt)/s(t), the bottom row of Figure 1 shows the relative (or fractional) rate of change of the trajectory of the power values or equivalently the rate of change of the trajectory of the log transformed power values. The results are described in terms of the relative rates of change. Theta power in males shows a relatively constant rate of decrease of about 8% to 9% per year from 12 to 16 years of age, then the rate of decrease diminishes in a fairly uniform way before stability is reached at about age 23. Theta power in females shows a relatively constant rate of decrease of about 2.5% to 4% per year from 12 to 16 years of age, then the rate of decrease
