Let $n$ be a positive integer. Let $P$ be the set of partitions of subsets of 
$\{ 1, 2, \ldots, n \}$ (so, for example, when $n=2$, the set $P$ contains $\emptyset, \{ \{1 \} \}, \{ \{2 \} \}, \{ \{1 \}, \{2 \} \}$, and $\{ \{1, 2 \} \}$). 

Turn $P$ into a poset by saying $\pi \preceq \sigma$ for $\pi,\sigma \in P$, if every part in $\pi$ is a subset of some part in $\sigma$.

What is the Mobius function of this poset (that is, for every $\pi, \sigma \in P$, I want to know if there is a simple formula which gives the value of $\mu_P(\pi,\sigma)$)?