In terms of the equations presented above, we could compare the distributions P (z|o, C) with P (z|n, C) using equations (5) or (16). However, Thater et al. (2010) and<cite> Dinu and Lapata (2010)</cite> both observe that contextualising both o and n can degrade performance; in view of this we actually compare P (z|o, C) with P (z|n) and make the further simplifying assumption that P (z|n) ∝ P (n|z). The similarity measure we adopt is the Bhattacharyya coefficient, which is a natural measure of similarity between probability distributions and is closely related to the Hellinger distance used in previous work on topic modelling (Blei and Lafferty, 2007) :