Friday, September 18, 2015

Designing Structure to Help People Prevent Errors

Are we making slips or mistakes?

Dr. Donald Norman (1988) presented two fundamental categories of errors that people make while interacting with work processes and systems:

  • Slips: …result from automatic behavior, when subconscious actions that are intended to satisfy our goals get waylaid en route.
  • Mistakes: …result from conscious deliberations
Norman warns us that the same mental processes that make us creative and insightful by allowing us to see relationships between apparently unrelated things, that let us leap to correct conclusions on the basis of partial or even faulty evidence, also lead to error.


All of us experience making slips each day. In fact the more skilled we are the more prone we are to simple slips. For example, when learning how to drive, we usually do not pull out in front of another vehicle without really paying attention. All of us as skilled drivers have probably experienced the horror of looking quickly and then pulling from the curb and almost being hit by another driver. People learning to drive will look two or three times, paying great attention and then only pull from the curb when the “coast is clear.” Slips are almost always small things and can be readily fixed. They are generally more embarrassing than dangerous. Norman has defined six types of slips:

 Types of slips:

  1. Capture errors
  2. Description errors
  3. Data-driven errors
  4. Associative activation errors
  5. Loss-of-activation errors
  6. Mode errors
Let’s consider each type of slip and an example in real life:

Capture Errors - A frequently done activity suddenly takes charge instead of the one you intended. The capture error usually happens when two different actions sequences have their initial stages in common. For example, it is a nice Saturday morning and your spouse asks you to drive to the store and pick up some orange juice for breakfast. You pull out of the driveway and start on your journey. The next thing you know you are pulling up to park at your workplace! The routine of driving to work has captured your intended action of driving to the store.

Description Errors - The intended action has much in common with others that are possible. As a result, unless the action sequence is completely and precisely specified, the intended action might fit several possibilities. For example, many of us who travel a lot have had the experience of coming from a client meeting, finding our rental car and then after some difficulty of attempting to get into the car, find we have the wrong car. Apparently, Ford has made several white Ford Taurus vehicles. Much to our embarrassment, we then start looking for the car we rented that morning.

 Data-Driven Errors - Automatic actions are data-driven, triggered by the arrival of the sensory data. But sometimes data-driven activities can intrude into an ongoing action sequence, causing behavior that was not intended. For example, while checking into a hotel, the clerk took a phone call from another guest who was calling for a wake-up call. The clerk then got off the phone and informed me that I would be in room 630. I mentioned to him, that earlier he said I would be on the first floor. He then confessed that he just took a call to wake a guest at 6:30 and had confused it with my room number.  I would be in room 110.

Associative Activation Errors
As we have noted, external data can trigger actions. Internal thoughts and associations can also lead to slips. For example, while watching TV the telephone rings on the show I am watching; not realizing the telephone ring is from the television show, I pick up my own phone and greet a dial tone! Associative activation errors can also come from associations between thoughts and ideas.  When we are in a social situation and are thinking we should not say something that might be inappropriate, and then to our shock and surprise we say it!

Loss-Of-Activation Errors

Many of us have committed this error and attributed to ourselves that age must be taking it toll on our mental abilities. Lack-of-activations errors occur because the presumed mechanism -- the activation of the goals, has decayed. For example, we walk from one room to another. Finding ourselves in the room, we are suddenly struck with the reality; why are we in this room? Looking around frantically, we cannot remember why we made the trip. We then go back to the room from whence we came to be “activated.” Looking around, we suddenly remember and then will renewed determination return to the other room to complete the action.

Mode Errors

Mode errors occur when devices have different modes of operation, and the action appropriate for one mode has different meanings in other modes. For example, on Microsoft Outlook I can make distribution lists to send certain types of communication to identified groups of people. A fantastic time saver! Periodically, I find it necessary to edit this list. When I select the entry to be deleted, I read from left to right and find the delete button and click. To my surprise, I have now deleted the entire list! The correct button is further to the right under “Members.” See Figure 1 for the example.
Figure 1: Mode Error Example on Outlook 
  In the next installments of this conversation we will cover the following topics:
    • Design lessons from the study of slips
    • More on the nature of mistakes
    • Creating structure to help people avoid slips and mistakes


  1. Psychology of Everyday Things, Donald A. Norman, Basic Books, 1988