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The many facets of errors (Design for Error – Part 2)

Slips make-up most of the everyday errors. You intend to do something and end up doing something else. Quite common. But what is interesting about slips, is the fact that experts are more prone to commit slips than novices.

Conscious Disregard

You got up late. You realize that there is an important meeting scheduled for that day and is starting earlier than usual. What is going on in your mind when driving to work? Many of us would be lying if following traffic ethics were to be the top priority. Should you slow down while taking a turn, should you wait for the red light to go green, especially when there appears to be no traffic? Obviously, most of the times we thank our lucky stars for scraping through the day.

Not everyone can, however, speak of that. Knowing fully well that one is throwing caution to the wind, workers continue with their usual work every day in many places. Why? Because the rules in some workplaces are so rigid, one will not be completing the tasks if they follow the rulebook to the last letter. Again, here the blame lies with the management. Making rules just to match the legal requirements will oftentimes make it illogical for the employees to follow them. And when things go south, it is the same employee who gets the blame. The employee who met the expectations while doing the same work in a similar mindset and environment. The responsibility lies solely with the higher-ups to determine why violations are happening if they are and work towards preventing such violations by modifying the workplace. A culture that encourages completion of tasks and meeting expectations above all else risks influencing oncoming errors, some of which can turn detrimental.

Slippery Mistakes

Whenever a person deviates from a generally accepted behavior they are said to have committed an ‘error’. This general acceptance of behaviors and actions can either be loosely based on something already in place and followed or can be determined after the fact. Don Norman and psychologist James Reason have developed a breakdown of errors. They are divided into two main categories: Slips and Mistakes. This has been since followed by studies of accidents and errors across industries.

Slips happen when the end goal of a task is correct but the undertaking action is not. In short, slips occur when the plan is correct but the execution is otherwise. Mistakes happen when the plan itself is wrong i.e. the end goal is incorrect. When someone intends to complete a task and miss a step that is when a ‘slip’ occurs. There are two kinds of slips: action-based slips and memory-based slips. Further classification can be done based on the cause of a said slip.

An action-based slip occurs when after pouring cereals and milk, you place the bowl in the refrigerator instead of the milk carton. A memory-based slip occurs when you forget to pour the milk and return the carton in the fridge. In the case of a mistake, since the goal itself is wrongly decided, any steps taken in its pursuit will become part of the mistake. Mistakes are classified as rule-based mistakes, knowledge-based mistakes, and memory-lapse mistakes.

Rule-based mistakes are committed when a correct diagnosis of a problem leads to the incorrect course of action. When the problem itself is misdiagnosed then a knowledge-based mistake is said to have taken place. A memory-lapse makes one forget the order of steps to follow. Consider the following example. You are preparing to visit a friend and have to use the map to reach the destination. A knowledge-based mistake is to get the destination wrong (maybe you used an old address). A rule-based mistake would occur when you took the wrong route although you had entered the destination correctly on the map. A memory-lapse would occur when you forget where to take next turn, a left or a right.

Error in Action

In terms of where each error type lies within our cognition levels, mistakes occur at a higher stage than slips. Actions occur between 8 stages. Once the goal is set, a plan is devised, replete with specifications. And the action is performed. A feedback from the environment is then perceived which, based on its interpretation will later be compared with the original plan. Slips can occur in the ‘lower’ stages of action i.e. during planning, performing, perception or interpretation. Memory-lapse-type mistakes can occur between any stages along the chain of action. Also, slips are subconscious while mistakes are conscious in nature. While we humans pride ourselves as creative beings, we can also be susceptible to finding relations between obscure objects. Summarizing a situation to get a simpler idea helps us understand a new environment easily. However, the same may sometimes nudge us to leave out critical information, thus contributing to a mistake.

A day in the life of slips

Slips make-up most of the everyday errors. You intend to do something and end up doing something else. Quite common. But what is interesting about slips, is the fact that experts are more prone to commit slips than novices. How you may ask. Experts carry on with their daily tasks in an automatic fashion, mostly handled in the subconscious state. Sure, you’re used to messaging in one hand and having food in the other. But you do tend to misspell a word here and there and blame autocorrect. Those who are new to a task will take considerable care in carrying them out, hence, run a lower risk of a slip.

Action slips happen due to at least one of the following causes:

Capture slips: Occurs when a part of the plan is repeated in two distinctive tasks, giving rise to confusion. So, on a weekend you start from your place and head towards a park. Except, mid-way you continue into the workplace. Why? Because it’s a single turn that differs between the two routes and you do spend more time in the office.

Description-similarity slips: These slips occur when different objects and/or systems have share similarity in their physical features. You may pick up the wrong carton of milk (different percentage of fat), while in a hurry, since every carton looks similar with a slight change in number on the pack.

Mode errors: Mode errors occur when using products that use similar controls for different actions. There are some lights in most homes that have multiple switches. Now when you switch off in a single switch, it might mean off or on depending on what the other switches are set at. Or a single knob in a music system can be used in different modes (cassette, CD, FM etc.) and will give different results in each mode. Confusion is bound to occur when you mistake the mode in which the system is set in. Oh, and we’ve all set the alarm to 6:00 PM!

Memory-lapse Slips: The most common of all slips. We have all forgot to collect a pen that we lent to a friend. Or, we might have been that friend who forgot to return. People lock the door by leaving the key inside. There are so many everyday examples of memory-lapse slips that we come across.

Design-wise many improvements can be made to provide a feedback to the user. These days, we do see some of the implementations as well. The refrigerator lets out a sound if the door is open for too long. At work, distinct tasks involving similar steps can be indicated by the change of environment, color codes and the related. A selected mode should be displayed so that we won’t miss to see it. Also, different shapes for knobs work wonders. The aircraft industry has implemented many of such changes to good effect. The cockpit is room to innumerable switches and controls. Imagine if two different variables are displayed in a similar fashion. If the angle of descent and the speed of descent are displayed in a similar fashion, things can go from bad to worse in the blink of an eye. Many accidents have occurred due to such anomalies, only for investigations to reveal the issue that is corrected in due time.


  • This series is a summary of Chapter 5 (Human Error? No, it’s Bad Design) from the book ‘The Design of Everyday Things’ by Don Norman

  • You can also listen to this insightful podcast by NPR Hidden Brain to understand how our minds work under stress –


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