The digital clock radio by my bed has a row of buttons across the top. The first four are labeled, from left to right, "Sleep", "Wake", "Hour", and "Minute". These are ordinary buttons that can be pressed, indistinguishable from each other except for their position and label. The fifth button, on the far right, is actually a sliding button that allows me to choose one of several settings to control the radio and the alarm. This fifth button is shaped just like the other buttons.

Let's consider a scenario that describes how someone might interact with my clock radio. (Thinking about such scenarios is something that all serious designers do.) In the evening I set the alarm, before I go to sleep. To do this I push the "Wake" button, and while I'm holding it down, I press the "Hour" and "Minute" buttons until I reach my wake-up time. I then push the sliding button to the "Alarm on" position, and I'm done, at least until the next morning. When the alarm goes off, I can either press the "Sleep" button to get a few more minutes of sleep, or change the sliding button setting to "Off".

Now for an equally important but less common scenario: I've just bought this radio, or perhaps there has been a power outage, and I'd like to set the time. To do this I simply press the "Hour" and "Minute" buttons and watch the numbers blink forward, in sequence, until the current time is reached. Now I'm done.

There are a few design issues worth pointing out.

First, the time-setting scenario is easier to carry out than the alarm-setting scenario: there are fewer buttons to press. In an effective design, the most common things that we do are the easiest to do, involving the fewest or easiest actions. After all, setting up a clock radio isn't something we do for fun. If setting the time is less common than setting the alarm, then it's fine to make the less common function relatively less accessible (as long as none of the functionality is hard to get to in some absolute sense.) The way my clock radio is designed, the opposite is the case.

Second, I've deliberately left out a few important factors from the first scenario that deal with my state of mind when I'm dealing with the clock radio. State of mind isn't always important, but it is here. How alert are you when you're awakened first thing in the morning by your alarm clock, compared with when you're getting ready for bed the night before? I imagine that I'm like most people: if I go to bed at a reasonable hour, setting the alarm is trivially easy; in contrast, the next morning, when the alarm wakes me up, I'm like a zombie. I reach out and feel around, usually before I open my eyes, until I find the button that turns off the alarm. Now remember that all the buttons look pretty much alike, and that you tell them apart either by remembering the order they're in or by reading the labels. Neither possibility can be taken for granted in this situation. Although I can usually manage to hit the correct button to turn off my alarm, occasionally I'll hit another button instead. These buttons just happen to be those that change the current time, either the hour or the minute. Ironically, correcting the time is one of the easiest procedures for this clock radio--fortunately for me, since I have to do this much more often than is really necessary.

We might summarize these lessons in this way. First, the things that you do most often should require the actions that are easiest and quickest to carry out (e.g., I can turn off the alarm or turn on the radio by using just one button). Second, it should still be possible to easily figure out how to do the things that you do less often, even though those actions may take more time. Third, when possible, it shouldn't take too much more time to undo an incorrect action than it did to carry it out in the first place (e.g., inadvertently changing the time is bad because it takes much longer to set it correctly again.) Fourth, if this not possible, then such inadvertent actions should be protected against (e.g., the way that changing the alarm requires holding down one button and then repeatedly pressing another.)