[kaoD]

> unless you're averaging them!

Nitpicking, but that seems like a great case for them being numbers. You will never average credit cards or zip codes. Averaging times is meaningful too (e.g. the mean arrival time).

I think op meant "adding" as generally doing math on them. Each quantity will have its own natural operations.

I see what you mean though, subtraction seems like a good heuristic by virtue of difference being mostly (always?) meaningful.