14. The system of claim 13 , where sampling the category label and the topic comprises sampling the category label and the topic from a distribution that satisfies: P ⁡ ( u d , i = c , z d , i = k | w d , i = v , w d , - i , z d , - i , u d , - i , y d , α , β ) = C v , k W ⁢ ⁢ T + β ∑ v ′ = 1 V ⁢ C v , k WT + V ⁢ ⁢ β ⁢ C k , c TC + α ∑ k ′ = 1 K ⁢ C k , c TC + K ⁢ ⁢ α ; where u d,i is a category label; c is a particular category;z d,i is a topic; k is a particular topic; w d,i is a sequence of one or more words; v is a particular sequence of one or more words; C v,k WT is a number of times that topic k is assigned to sequence v , not including a current instance of sampling i; C k,c TC is a number of times that topic k is assigned to category c, not including the current instance of sampling i; α is a constant; β is a constant; and d is a document.
14. The system of claim 13 , where sampling the category label and the topic comprises sampling the category label and the topic from a distribution that satisfies: P ⁡ ( u d , i = c , z d , i = k | w d , i = v , w d , - i , z d , - i , u d , - i , y d , α , β ) = C v , k W ⁢ ⁢ T + β ∑ v ′ = 1 V ⁢ C v , k WT + V ⁢ ⁢ β ⁢ C k , c TC + α ∑ k ′ = 1 K ⁢ C k , c TC + K ⁢ ⁢ α ; where u d,i is a category label; c is a particular category;z d,i is a topic; k is a particular topic; w d,i is a sequence of one or more words; v is a particular sequence of one or more words; C v,k WT is a number of times that topic k is assigned to sequence v , not including a current instance of sampling i; C k,c TC is a number of times that topic k is assigned to category c, not including the current instance of sampling i; α is a constant; β is a constant; and d is a document. 15. The system of claim 14 , wherein performing the plurality of sampling iterations comprises performing a respective portion of the sampling independently on each of multiple processors, and wherein each processor p calculates an update ΔC |p WT and an update ΔC |p TC , and C TC k,c and C v,k WT are updated according to the expressions C WT += ∑ p = 1 P ⁢ Δ ⁢ ⁢ C | p WT and C TC += ∑ p = 1 P ⁢ Δ ⁢ ⁢ C | p TC .