Clinical Systems: Usability

Clarifying the Metrics of Usability is a paper that I found while trolling the web this morning.  I'm looking for a way to clarify to a software vendor how we expect their product to be usable.  It's slippery.  We want to contractually define that the product has to meet the user's needs and expectations.  Of course this is the goal of the vendor too … but they often seem to miss the mark.  So often – software isn't as usable as it should/could be, and this is terribly important.  Yet it's so hard to define or measure that the standard "I know it when I see it" just won't do in the contract.