I have consecutive measurements of two different phenomena (x and y) on the same subject, giving me a collection of time-series x_ik(t) and y_ik(t), where i is the i-th measurement and k is the k-th subject. I would like to compare the variability across k of x and y to inquiry what of the two phenomena is more variable. Is there a formal way of doing it? Thanks!
EDIT: I figured that if I take the variance across i-s at each time sample, I run into a problem. The signal (x or y) that is more "steep" will have a higher variability because the variance is computed along the vertical axis. Is there a known way to solve this?