500 features were sampled from the top 20,000 unigrams 4 , 2000 social media LDA topics (Schwartz et al., 2013a), and all 64 categories from the LIWC lexica (Pennebaker et al., 2007).
note that at higher levels of analysis (at user and county levels) as the distribution of features becomes closer to Normal, modeling features as Bernoulli is clearly sub-optimal where as at the message level modeling unigrams as a Bernoulli is superior.