Keep It Short

An error message should be clear, consistent, and specific, but you also want to keep it as brief as possible. Why? Because you want the user to read it, and users don't like to read long, involved error messages. The most effective error message is a short error message.

Let's look at a typical example. Suppose while renaming a file a user types in an invalid filename character, such as a colon. You could issue the following error message:

Aside from being incredibly long, this error message accomplishes all the other goals we have outlined so far. Its information is technically accurate, and it completely informs the user what the problem is and how to correct it. But it's way too much. Do you want to read it? I don't. An error message like this begs not to be read.

Now let's look at a much better error message used by Windows Explorer:

This error message is much more to the point, and it also accomplishes all our goals, including being very specific. This error message makes it clear that the user entered an invalid character and implies a solution to fix the problem by removing the invalid characters. Note that this message also avoids typical technical terms such as error or invalid. Lastly, note that this example shows that if it is awkward to describe valid input, it might be easier to describe invalid input. (The opposite is also true.)

Aside from their length, there is another significant difference between these two error messages. The first error message tries to train the user about filenames, and the second version doesn't bother. Interestingly, of the many attributes of a good error message, the message's ability to train the user isn't one of them. Why? Because this goal would be in direct conflict with keeping the message short. Also, attempting to train the user with an error message is doomed to failure anyway. When a user receives an error message, his immediate objective is to solve the problem and get back to work. The user isn't likely to be in the mood to take time out for a lesson on how to use the software.