Not Missing at Random

Not Missing at Random (NMAR) is data that is missing for a specific reason..

Here is an interesting example of NMAR data.. with the message that one shouldn't be sad and low, after reading on Facebook, about abnormally flattering lives of their friends' ..

http://simplystatistics.org/2014/01/17/missing-not-at-random-data-makes-some-facebook-users-feel-sad/