The VI is the sum of these conditional entropies.
given a corpus labeled with hidden states and POS tags, if p(y),p(t) andp(y, t) are the empirical probabilities of a hidden state y, a POS tag t, and the cooccurance of y and t respectively, then the mutual information I, entropies H and variation of information VI are defined as follows: As Meilǎ (2003) shows, VI is a metric on the space of probability distributions whose value reflects the divergence between the two distributions, and only takes the value zero when the two distributions are identical.