EM算法(Exception Maximization Algorithm)介绍
2017-08-30 11:50
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算法介绍
本文以隐马尔可夫链的学习问题为例来简单介绍下EM算法。假设隐马尔可夫模型的观测序列已知(记Y为观测变量),需要求得模型的初始状态π,隐状态转移矩阵A和观测状态转移矩阵B,上述参数合记为θ=(π,A,B)。所求参数θ可以表述为最大化似然函数L(θ):(式1)argmax θ L(θ)=logP(Y|θ)
然而logP<
20000
/span>(Y|θ) 并不能直接求得,我们记P(Y,Z|θ)P(Z|Y,θ i ) =δ i (θ) , 则有δ i (θ i )=P(Y|θ i ) 。
(式2)E Z|Y,θ i [logδ i (θ)]=∑ Z P(Z|Y,θ i )logP(Z,Y|θ)P(Z|Y,θ i )
注意P(Z|Y,θ i )=P(Z,Y|θ i )/P(Y|θ i ) , 上式可写为:
(式3)E Z|Y,θ i [logδ i (θ)]=∑ Z P(Z|Y,θ i )logP(Z,Y|θ)P(Z,Y|θ i ) +∑ Z P(Z|Y,θ i )logP(Y|θ i )
把E Z|Y,θ i [logδ i (θ)] 看作是关于θ的函数,则上式后一部分的值为∑ Z P(Z|Y,θ i )logP(Y|θ i )=logP(Y|θ i )=L(θ i ) 与变量θ无关,可看作是常数。
迭代法求取式1的EM算法可表述为
随机选取初始化的θ=θ 0
Exception步: 求Q(θ,θ i )=E Z|Y,θ i [logδ i (θ)]−L(θ i )=∑ Z P(Z|Y,θ i )logP(Z,Y|θ)P(Z,Y|θ i )
Maximization步:θ i+1 =argmax θ Q(θ,θ i )
不断重复E步和M步直到θ收敛。
收敛性简单证明
由Jensen’s inequality有:E Z|Y,θ i [logδ i (θ)]≤logE Z|Y,θ i [δ i (θ)]
注意到
logE Z|Y,θ i [δ i (θ)]=log∑ Z P(Z|Y,θ i )P(Z,Y|θ)P(Z|Y,θ i ) =log∑ Z P(Z,Y|θ)=logP(Y|θ)=L(θ)
Q(θ i ,θ i )=E Z|Y,θ i [logδ i (θ i )]−L(θ i )=∑ Z P(Z|Y,θ i )logP(Z,Y|θ i )P(Z,Y|θ i ) =0
则有
L(θ i+1 )=logE Z|Y,θ i [δ i (θ i+1 )]≥E Z|Y,θ i [logδ i (θ i+1 )]=Q(θ i+1 ,θ i )+L(θ i )
而θ i+1 =argmax θ Q(θ,θ i ) , 故Q(θ i+1 ,θ i )≥Q(θ i ,θ i )=0
因此始终有L(θ i+1 )≥L(θ i ) 成立。
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