加1平滑 ·最简单,但不是真的能用 T训练数据,V词表w词 预测p'(wh)=(c(h,w)+1)/(c(h)+V) 特别非条件分布时p(w)=(c(w)+1)(T+V) 问题:经常会V>c(h),甚至v>>c(h) 举例:T<s> what is it what is small!?=8 V=what, is, it, small,? <s>, flying, birds, are, a, bird,,V=12 p(it)=0.125,p(what)=0.25,p()=0,p( what is it?=0252*0.2520.001 p( it is flying)=0.125*0.25*02=0 p(it)=0.1,p(what)=0.15p()=0.05,p( what is it?)=0.152*0.12≈0.0002 p( it is flying)=0.1*0.15*0052≈00004加1平滑 • 最简单，但不是真的能用 – T:训练数据,V:词表,w: 词 预测 p’(w|h)=(c(h,w)+1)/(c(h)+|V|) 特别:非条件分布时p’(w)=(c(w)+1)/(|T|+|V|) – 问题：经常会|V|>c(h)，甚至|V|>>c(h) • 举例：T: <s>what is it what is small? |T|=8 – V={what,is,it,small,?,<s>,flying,birds,are,a,bird,.}, |V|=12 – p(it)=0.125, p(what)=0.25, p(.)=0, p(what is it?)=0.252*0.1252≈0.001 p(it is flying.)=0.125*0.25*02=0 – p’(it)=0.1, p’(what)=0.15,p’(.)=0.05, p’(what is it?)=0.152*0.12 ≈0.0002 p’(it is flying.)=0.1*0.15*0.052 ≈0.00004