Why Priors Matter

The conclusion of this paper is that it is best to use an asymmetric alpha and a symmetric beta. I love its interpretation. “An asymmetric Dirichlet prior over theta serves to share commonalities across documents and a symmetric Dirichlet prior over phi serves to avoid conflicts between topics.”

Why Priors Matter

2 thoughts on “Why Priors Matter

  1. Alice says:

    Okay, I think I understand the symmetric betas serving to avoid conflicts among topics. I don’t quite get how asymmetric alphas serving to share commonalities across documents. I will have to read the paper again.

    How are you doing? Is everything okay?

    1. zukjimote says:

      Well, the proportions of alpha is the expected value of theta~Dir(alpha). If alpha is asymmetric, then some elements of theta are expected to be bigger than the others, which means some topics are expected to have higher probabilities than other topics. These topics become the common topics across documents, because they have a higher chance to occur in every document. The paper shows that these common topics usually turn out to be function words.

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