Rubin (1976, and elsewhere) claimed that there are three kinds of “missingness”: missing completely at random; missing at random; and missing not at random. He gave examples of each. The article that now follows takes an opposing view by arguing that almost all missing data are missing not at random.
Knapp, T. R. (2019). Almost all missing data are MNAR. Journal of Modern Applied Statistical Methods, 18(2), eP3523. doi: 10.22237/jmasm/1594045320