Actually, since you are using period as your class variable, PROC SUMMARY will essentially ignore "period" as one of the _numeric_ variables and will issue the following:
WARNING: Variable period already exists on file WORK.MYSUMMARY.
WARNING: The duplicate variables will not be included in the output data set of the output statement number 1.
If your original is doing most of what you want all you actually want to do is ignore the variable then drop it from analysis:
proc summary data=mydata (drop=period) nvar;
Actually, since you are using period as your class variable, PROC SUMMARY will essentially ignore "period" as one of the _numeric_ variables and will issue the following:
WARNING: Variable period already exists on file WORK.MYSUMMARY.
WARNING: The duplicate variables will not be included in the output data set of the output statement number 1.
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