Thanks for various replies, they are helping. Please keep sending ideas.

This is such a common situation there is a method used in history and genealogy research called "Discrepancy Charts". A brief article at http://genealogy.com/37_neill.html titled "Organize the Inconclusive with Discrepancy Charts" gives a brief explanation and tabular chart form that may be useful to SMW community when addressing this frequent pattern.

Best regards,
Michael Skelly