59. *Let X, denote the observation for the ith subject at time 1. Let Y, denote the observation for the ith subject at time 2. Let D; Y;- X;.

a) Letting X, Y. and D denote the means of these observations, show that =- X.

b) Is the median difference (i.e.. the median of the D; values) equal to the difference be- tween the medians of the Y, and X, values? Show this is true, or give a counterexample to show that it is false.