Table 1.

Patterns of missing data

Missing data pattern	Description
Missing completely at random (MCAR)	• Data are missing for reasons not related to observed or unobserved variables • Simple statistical approaches to deal with missing data can provide unbiased results
Missing at random (MAR)	• Probability of missingness is related to observed data but not to unobserved data • MAR is the assumption for most imputation methods
Missing not at random (MNAR)	• Missingness is related to unobserved data

It is not possible to distinguish between MAR and MNAR based on observed data