There are two forms of randomly missing values:
MCAR or Missing completely at random. Such errors happen when the missing values are randomly distributed across all observations.
We can confirm this error by partitioning the data into two parts –
- One set with the missing values
- Another set with the non-missing values.
After we have partitioned the data, we conduct a t-test of mean difference to check if there is any difference in the sample between the two data sets.
In case the data are MCAR, we may choose a pair-wise or a list-wise deletion of missing value cases.
MAR or Missing at random. It is a common occurrence. Here, the missing values are not randomly distributed across observations but are distributed within one or more sub-samples. We cannot predict the probability from the variables in the model. Data imputation is mainly performed to replace them.