I'm looking at a specific derivation on wikipedia relevant to statistical mechanics and I don't understand a step.
$$ Z = \sum_s{e^{-\beta E_s}} $$
$Z$ (the partition function) encodes information about a physical system. $E_s$ is the energy of a particular system state. $Z$ is found by summing over all possible system states.
The expected value of $E$ is found to be:
$$ \langle E \rangle = -\frac{\partial \ln Z}{\partial \beta} $$
Why is the variance of $E$ simply defined as:
$$ \langle(E - \langle E\rangle)^2\rangle = \frac{\partial^2 \ln Z}{\partial \beta^2} $$
just a partial derivative of the mean.
What about this problem links the variance and mean in this way?