Microsoft Excel has a lot of great features. But I’ve long wondered why, when you type an equation in a cell, the default is to treat it like text.

For example, when I type 1+2 in a cell, Excel doesn’t add the two numbers, but instead displays “1+2” in the cell.

Now, this is the kind of behavior I expect in a word processing or presentation program. But Excel is a spreadsheet. It’s made specifically for processing numbers. So why did the designers decide that the default should be to treat an equation like text unless I type = or + at the beginning?

In contrast, Corel’s Quattro Pro treats numbers like numbers. When I type 1+2 in a cell in Quattro Pro, I get the result I expect: 3. I don’t have to type an unnecessary symbol at the beginning of the equation to signal that I’m using numbers.

Excel’s counter-intuitive design calls to mind advice from a fellow designer: “Don’t design to the exception; design to the rule.” Her advice is good. As designers, we shouldn’t force users to jump through hoops constantly because we’ve designed to the exception.

However, are there times it is appropriate to design to the exception? Accomodating colorblindness comes to mind. I once designed an interface that relied on color to provide important cues to the user—only to discover that one of our key stakeholders was colorblind.

So how many users are colorblind? I’ve seen estimates ranging from 5 – 8% for males, and less than 1% for females. That means we’re talking relatively small percentages here. Is that too small to design to the exception?

I don’t think so. And here’s the key difference between the Excel example and the colorblind example: in Excel, the exception was an extremely unlikely user scenario. Users virtually never want “1+2” to appear in Excel as “1+2.” They want to see the results of the equation. It doesn’t make sense to design to a scenario that almost never happens.

On the other hand, when we design to the exception for colorblindness, we are designing for a user, not a scenario. Most users probably won’t have challenges because of color in an interface. But for those who do, it affects their entire experience. Not only that, there are alternatives to color that work equally well for either type of user, colorblind or not. So in this case, it makes sense to design to the exception and find a win-win solution for the majority of users.

So when we question whether or not to design to the exception, we can make a better choice by asking if we’re designing for a scenario or for a user.