Conditional entropy of finite measurable partitions
= Conditional entropy of finite measurable partitions
{title2=$H_\mu(\xi\mid\eta)$}
For finite partitions $\xi,\eta$,
$$
H_\mu(\xi\mid\eta)
=\sum_{B\in\eta}\mu(B)H_\mu(\xi\mid B)
=H_\mu(\xi\vee\eta)-H_\mu(\eta).
$$
<Conditioning reduces entropy>, so $H_\mu(\xi\mid\eta)\leq H_\mu(\xi)$.