Currently I am reading other people's coding. Just want to know how the SAS converts the output dataset from vertical to horizontal in the proc means procedure. The details are in the attachment I uploaded.
This is done internally by PROC MEANS. We don't know what the internal code is. We don't know how SAS does this conversion.
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Thank You to PaigeMiller for the reply. This means that I have to remember it for the special treatment to rearrange the data.