Define topic entropy to measure the breadth of a conference:
H ( z | c , y ) = − ∑ i = 1 K p ^ ( z i | c , y ) l o g p ^ ( z i | c , y ) {\displaystyle H(z|c,y)=-\sum _{i=1}^{K}{\hat {p}}(z_{i}|c,y)log{\hat {p}}(z_{i}|c,y)} .