Current Knowledge is a much better way to think about the problem.
Consistency in design is about making elements uniform — having them look and behave the same way. We often hear designers talk about consistent navigation, consistent page layouts, or consistent control elements. In each case, the designer is looking for a way to leverage the usability by creating uniformity. After all, if the user learns to operate the design in one place, why not have that knowledge transfer to the next. This is all good. But wrong.
For example, let’s look at a design decision our friends at Avis.com once made. At the time, they chose to use the asterisk (*) to denote optional fields instead of denoting mandatory fields. Less than 10% of the fields on the entire site are optional. If they were consistent with the outside world, entire forms would have every field denoted with an asterisk. That might create its own problems.
So, Avis.com’s designers did what designers are supposed to do: they made a choice. They chose to be inconsistent with practically every other form on the web. But they were internally consistent with themselves.
The problem with thinking in terms of consistency is that those thoughts focus purely on the design and the user can get lost. “Is what I’m designing consistent with other things we’ve designed (or others have designed)?” is the wrong question to ask.
Instead, the right question is, “Will the user’s current knowledge help them understand how to use what I’m designing?” Current knowledge is the knowledge the user has when they approach the design. It’s the sum of all their previous experiences with relevant products and designs.
If the designers at Avis had asked, “Will the user know how to separate optional fields from mandatory ones?” they would likely have come to the conclusion that using asterisks to denote optional fields might confuse a user or two. They could’ve used a different typographic or visual solution, or resorted to the old-tried-and-true “(optional)” text next to the appropriate field label.