PROC CAUSALMED stops processing if you specify the REF= option in the CLASS statement. As a result, the following message occurs in the log:

ERROR: Invalid reference value for XYZ

This problem occurs even when you specify the reference level correctly or when you specify the keyword FIRST or LAST.

Workaround

To circumvent this issue, specify any of the following CLASS statement options:

• the REF= global-option
• a DESCENDING individual CLASS variable option 
• an ORDER= individual CLASS variable option 

Here are some examples of the syntax that you can use:

• CLASS variable variable /REF=FIRST;
• CLASS variable (DESCENDING) variable ; 
• CLASS variable variable (ORDER=FORMATTED);