We assume that π is a K-vector (where we take the limit as K → ∞) where π k describes the population proportion of documents that contain latent feature k. We suppose that π is generated by the stick-breaking construction (Doshi-Velez et al., 2009).
the random assignment of texts to individuals induces a distribution over a probability measure on treatment vectors Z, f (Z) = this requires that every combination of treatment effects is possible from the documents in our corpus.