A binary indicator x, which is generated by a Bernoulli distribution π x drawn from a Beta distribution with symmetric hyperparameter γ x , determines whether a word is generated by ψ z or ψ e .
if x = 0, a word is drawn from ψ z ; otherwise the 3 http://en.wikipedia.org/wiki/July 2009 Urumqi riots Figure 6: The temporal distribution of documents about the Urumqi riots, which can be described by a Gaussian distribution (the dashed curve).