Ahh... you know, I actually had the mechanics straight, it was the fine details I screwed up. Somehow in my head I managed to lump the one excluded left record in with the duplicate 1s and 3s, instead of the 2s and 4s where it belonged, if that makes sense. So I subtracted one of the duplicates rather than NULLing the appropriate record. At least I understood the concept. Forest for the trees, etc.

thanks!

ron