1. 标题：Identification and Estimation of Local Average Treatment Effects
    作者：Guido W. Imbens and Joshua D. Angrist
    出版社：Econometrica, Vol. 62, No. 2 (Mar., 1994), pp. 467-475
    电子链接：http://www.jstor.org/pss/2951620
2. 标题：Maximum likelihood estimation for mixed continuous and categorical data with missing values
    作者：RODERICK J. A. LITTLE and MARK D. SCHLUCHTER
    出版社：Biometrika 1985 72(3):497-512; doi:10.1093/biomet/72.3.497

    电子链接：http://biomet.oxfordjournals.org/cgi/content/abstract/72/3/497