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Dan Iaboni

Dan Iaboni

Ph.D - Principal Experience Architect

Not All Errors Are the Same

While driving the other day I tried to switch the radio station using the controls on my steering wheel.  Instead, I accidentally redialed a friend. Depending on the mode, the controls that are used to control the radio stations are the same controls that are used to control the phone.It wasn’t a big deal, I just hung up and switched the mode so that I could complete my task, but I made an error.

We often hear or use the expression “human error” to describe how an accident occurred, but human error is a term that encapsulates a few different types of errors. There are accidental errors that are execution errors, resulting from an incorrect action, or planning errors, resulting from incorrect decision making. There are also intentional errors, called violations, where the individual decides to act inappropriately.

When you know what you need to do to complete a task, but in the process of completing the task you hit the wrong button or hit the right button but the system was in the wrong mode, (like what happened to me), or you just forgot to perform one of the steps, then you have made an execution error. There are two types of execution errors: slips and lapses.

Slips result from failures in user attention. For example, if a user intends to type in the postal code A1A 2X3, and types S1S 2X3 instead, two slips occurred: hitting the incorrect key, and failing to notice the incorrectly entered value. Although it sounds harmless, a slip can have just as dire consequences as a mistake or lapse. ValueJet flight 592 crashed into the Everglades because the pilots made a slip by focusing their attention on a blinking LED, rather than looking at other gauges and readouts.

Lapses result from errors in user memory. So going back to the address field example, the user fills out the address, but forgets or misses the entry for the postal code field. Lapses typically occur while the user is performing their actions, and they either forget a step or lose track of what they are doing.

Errors in planning are called mistakes and result from problems with rules or knowledge.  A rule-based error results when the user selects a bad rule to follow or incorrectly applies the correct rule. For example, deciding to pass someone by entering oncoming traffic when there is not enough time to complete the pass, is an incorrect rule selection. A mistake of knowledge stems from lack of information about the situation or confirmation bias with the information that is present.  Mistakes typically occur due to the complexity of the task reducing situation awareness, time pressures, or lack of training or experience.

Errors in action and planning are accidental errors. An intentional error, where the user knowingly breaks a rule, is known as a violation.  To design for violations, designers need to understand why the user is knowingly performing the task incorrectly, (e.g. are they taking a shortcut because they know a faster way of doing things or are they just trying to avoid work), and then needs to redesign to support the actual user behaviour. In cases where violations can create unsafe conditions, then the designer needs to find ways to constrain or discourage the users.

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When we conduct usability tests we are watching for participants to make errors. Understanding the type of error users make helps us understand how to redesign to prevent or reduce the likelihood of the error reoccurring.  If users are making a large number of slips, then the designer needs to take steps to address capturing or maintaining the user’s attention during usage, or design to reduce the amount of attention required.

Mistakes can be reduced by providing improved decision-making support to users.  We may not be able to eliminate all human error from our designs, but we can design to reduce the impact of making an error.
Dan Iaboni

Dan Iaboni

Ph.D - Principal Experience Architect

Dan firmly believes that technology must be created with the user in mind. Never shy to critique a bad design, Dan uses the Akendi blog to shine a spotlight on usability mistakes…and their solutions. Leveraging his background in engineering, computer science, psychology, and anthropology, Dan offers a unique perspective on the latest UX trends and techniques.


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Akendi is a human experience design firm, leveraging equal parts experience research and creative design excellence. We provide strategic insights and analysis about customer and user behaviour and combine this knowledge with inspired design. The results enable organizations to improve effectiveness, engage users and provide remarkable customer experiences to their audiences.