I tried: I_MENOP_NA00_OOPH_DT_3994 eq "" but it does not work for a partial date type field.
<RuleDef OID="MENOPAUSAL_STATUS_V21RULE12" Name="OOPH Provided But Procedure Date Not Provided">
<Description>If OOPH is marked Yes, the Procedure Date should be provided.</Description>
<Expression>I_MENOP_NA00_OOPH_9377 eq 1 and I_MENOP_NA00_OOPH_DT_3994 eq ""</Expression>
Ok. So I found out that for the software I am using (OpenClinica) at least, there is no way to write rule against a partial date type field. I would have to create the field as a regular date or simply not have any rules against it.
I wil have to change the case report form design...
Just wanted to let you guys know, in case someone have the same problem.