This is a nice summary of the advantages and disadvantages of various methods for handling missing values.
Pigott TD. A Review of Methods for Missing Data. Educational Research and Evaluation. 2001;7(4):353-383. doi:10.1076/edre.7.4.353.8937. Available at<U+00A0> http://www.stat.uchicago.edu/~eichler/stat24600/Admin/MissingDataReview.pdf
<U+00A0>