Single imputation 就是 replace each missing value with a single number, 这个single number可以是column的mean、medium,甚至是从其他非missing value随机出来的数字(到此的方法,都是针对于complete random missing才适用),还可以是column中其他非missing value拟合出来的值,eg: regress missing value in X_j on X_-j;.
Multiple imputation:也是regress missing value in X_j on X_-j,但是要plus noise and repeated for several times. <http://www.ats.ucla.edu/stat/sas/library/multipleimputation.pdf>