Error Tolerance in Usability Design

Julian Abich, Ph.D., Senior Human Factors Engineer

Let's talk about error tolerance. I'm not talking about dealing with people who annoy you or what your parents practiced when they raised you (although similar principles probably apply). I'm talking about the flexibility of a system to continue to function in the presence of an error.

Why would this be a good thing? Why do I want to use something possibly broken? In usability design, it's more about allowing users to achieve success, without being precise. Imagine if you misspelled something during your Google search and "Zero Search Results" appeared. Or if you were looking for a specific airline's website, but in return, you get a link to a mathematical definition. How quickly would you abandon the use of that tool? Has frustration kicked in? How would this impact businesses that depend on online traffic? (BTW, Google tried experimenting globally with Zero Search Results and, as you can imagine, angered many users). Error-tolerance can help your product be more usable.

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Why would error-tolerance be a bad thing? You'd want low error tolerance in a manufacturing setting (usually called fault tolerance in this context). "Why is that?" you say. Well, I won't point any fingers, but some recent events in the aviation industry have left a fairly young fleet of aircraft grounded after some pretty scary incidences (luckily no one was severely injured). While we don't know the exact cause of these problems, some initial investigations indicate something to do with missing bolts on a door. Error tolerance for a technician assembling an airplane should be very low, but humans are prone to making errors. So, how do we account for humans making errors? We dummy-proof the process. Using the door bolt example, imagine if the number of bolts needed for the aircraft came in a specific package just for that door. Meaning every single bolt in that package has to be used for that door to be properly installed. The bolts in the package could be laid out on a visual graph representing where they go on the door to provide more cues to the technician. There could even be a counter on the tool to keep track of how many bolts were installed. This way, if one bolt is left in the package, it is clear something is wrong and must be fixed. What we have done is removed the cognitive burden from the human, provided the needed information within the environment, and improved the success of completing the task correctly. This is called behavior-shaping constraints. The result is a cheap, simple usability solution that can save thousands of lives and millions of dollars.

What did we learn? If done well, error tolerance can keep the user happy when interacting with your product. It can help users succeed, even when they have never used your product. If error tolerance is overlooked, it can lead to catastrophic outcomes when the system fails. We also learned that planes can fly without doors.