Table 1. Common examples of the three missing data mechanisms in clinical trials.

Missing Mechanism	Examples
MCAR	Administrative censoring: follow-up is terminated because the study has ended.
	Migration-study participants move and are unable to complete visits.
	Random failure of the experimental instrument (e.g. test tube break, equipment failure)
MAR	Missing data caused by features of the study design such as participants being removed from the trial if their conditions are not controlled sufficiently well according to protocol criteria.
	Dropout based on recorded side-effects.
	Dropout based on known baseline characteristics.
MNAR	Dropout based on the unobserved response (e.g., a person not responding to treatment is more likely not to provide an observation).
	Participants miss a visit because they’ve had an outcome.