<P> In statistics, missing data, or missing values, occur when no data value is stored for the variable in an observation . Missing data are a common occurrence and can have a significant effect on the conclusions that can be drawn from the data . </P> <P> Missing data can occur because of nonresponse: no information is provided for one or more items or for a whole unit ("subject"). Some items are more likely to generate a nonresponse than others: for example items about private subjects such as income . Attrition ("Dropout") is a type of missingness that can occur in longitudinal studies - for instance studying development where a measurement is repeated after a certain period of time . Missingness occurs when participants drop out before the test ends and one or more measurements are missing . </P>

Missing at random versus missing completely at random