Replaces missing values with the mean for the entire series.
Mean of nearby points
Replaces missing values with the mean of valid surrounding values. The span of nearby points is
the number of valid values above and below the missing value used to compute the mean.
Median of nearby points
Replaces missing values with the median of valid surrounding values. The span of nearby points
is the number of valid values above and below the missing value used to compute the median.
Linear interpolation
Replaces missing values using a linear interpolation. The last valid value before the missing
value and the first valid value after the missing value are used for the interpolation. If the first
or last case in the series has a missing value, the missing value is not replaced.
Linear trend at point
Replaces missing values with the linear trend for that point. The existing series is regressed
on an index variable scaled 1 to n. Missing values are replaced with their predicted
values.