Filter query from access form using variable

This is called cascading combo boxes.  In order to make this work, you must have a table where you have both sets of values.  You need to set the RowSource for the 2nd combo to:

SELECT DISTINCT Measure
FROM qry_PDM_Measures
WHERE [Project Category] = Forms!frm_PDM_Results.cboProjectCategory

You then need to add code two two events, the first is to the cboProjectCategory_AfterUpdate event.  it should look like:

Private Sub cbo_ProjectCategory_AfterUpdate

    me.cboMeasure.Requery

End Sub

The other is in the Form_Current event.  This will make sure that if you go to a record that already has values in those fields, the combo box will display the correct list of items for the already selected Project Category

Private Sub Form_Current

    me.cboMeasure.Requery

End Sub