做下一筆記
wiki里面的定義 http://en.wikipedia.org/wiki/Latent_Dirichlet_allocation
關鍵所在：it posits that each document is a mixture of a small number of topics and that each word's creation is attributable to one of the document's topics。
將文檔看成是一組主題的混合，詞有分配到每個主題的概率。
Probabilistic latent semantic analysis（PLSA） LDA可以看成是服從貝葉斯分布的PLSA