To create higher frequency estimates, specify the input and output intervals with the FROM= and TO= options, and list the variables to be converted in a CONVERT statement. For example, suppose variables X, Y, and Z in the data set ANNUAL are annual time series, and you want monthly estimates. You can interpolate monthly estimates by using the following statements:
proc expand data=annual out=monthly from=year to=month; convert x y z; run;
Note that interpolating values of a time series does not add any real information to the data as the interpolation process is not the same process that generated the other (nonmissing) values in the series. While time series interpolation can sometimes be useful, great care is needed in analyzing time series containing interpolated values.