EM algorithm (Dempster et al.1977)，a widely used approach to learning in the presence of unobserved variables. The EM algorithm can be used even for variables whose value is never directly observed, provided the general form of the probability distribution governing these variables is known.
其实吧，EM算法就是生活我们称秤的时候，来回拨及逐渐优化的过程，呵呵！