Missing Completely at Random
Missing completely at random (MCAR) is the only missing data mechanism that can actually be verified. Missing data are MCAR when the probability of missing data on a variable is unrelated to any other measured variable and is unrelated to the variable with missing values itself. In other words the missingness on the variable is completely unsystematic. For example when data are missing for respondents for which their questionnaire was lost in the mail. This assumption can be tested by separating the missing and the complete cases and examine the group characteristics. If characteristics are not equal for both groups, the MCAR assumption does not hold.