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If the observations are not in the order that you want, you must either sort the data set or create an index for it before using BY-group processing.
If you use the MODIFY statement in BY-group processing, you do not need to presort the input data. Presorting, however, can make processing more efficient and less costly.
You can use PROC SQL views in BY-group processing. For complete information, see the SAS Guide to the SQL Procedure: Usage and Reference.
SAS/ACCESS Users: If you use views or librefs, refer to the SAS/ACCESS documentation for your operating environment for information about using BY groups in your SAS programs.
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