Human Factors 101


Human Factors 101

Affordances and Constraints Improve Reliability

In the first article in this series, we introduced concepts of human factors engineering (HFE) and their application to healthcare. We discussed how healthcare traditionally relies on the “weak aspects of cognition” (short term memory, attention to details, vigilance, multitasking etc.) and how that contributes to many of the errors experienced in healthcare. The first of several concepts of HFE is to make things visible. In this article, we will take a deeper look at two more concepts: affordances and constraints.

The term “affordance” is used in a variety of fields and has taken on a number of definitions depending on the context in which it is used. The term was first introduced by psychologist James Gibson (1977) in “The Theory of Affordances” and was defined as an action that an individual can potentially perform in his or her environment. These actions were independent of the individual’s ability to recognize them. In The Design of Everyday Things (1990), Donald A. Norman, professor emeritus of cognitive science at University of California, San Diego, focused the definition much more on interaction possibilities that we can perceive, which he later termed a “perceived affordance” (PA). In HFE, clinicians care much more about what they perceive than what is actually true. We focus on how a device or object “communicates” to us how to interact with it. The concept not only relies on the physical characteristics of the object but also is affected by the goals and past experiences of the clinician. Imagine a wheel barrel, the way the handles, wheel and container portion are constructed convey messages about possible uses, actions, and functions. Combine those attributes with an individual’s goal to move things, and he or she will easily conceptualize how to grasp each handle, tilt the wheel barrel on the wheel, and push it. Knobs are for turning, buttons are for pushing; well-designed equipment should require little instruction.

As healthcare providers, we are constantly expected to interact with new equipment. Suppose you have a new “smart pump” with a touch screen. There may be many pixels on the screen but touching many of them will lead to no action. To help the clinician perceive which pixels to touch to lead to an action, those pixels can be created in a three-dimensional representation so the clinician will perceive that touching this pixel will lead to an action.

Closely tied to the use of affordances is the need for timely feedback. Many errors arise from mental slips or lapses that occur without good feedback; we have no idea that an error has occurred. Remember the old telephone systems that had buttons that made a sound when pushed so you knew you had successfully pressed the button. The feedback was immediate. With many of the newer digital phones, no sound is associated with pressing the button, and you are not sure whether the action was successful. Look around your clinical unit. You will see the concept of immediate feedback utilized extensively: red and green diode lights indicate that a connection is correct or not, alerts in the computer system show if the imputed order is outside a predetermined range, etc. Interventional cardiologists use feedback by utilizing catheters with different textures so they can determine which one they are about to manipulate without looking at it.

In contrast to an affordance, a constraint limits the number of choices or actions the clinician can choose. Instead of expecting the provider to choose the correct option each time, a variety of constraints can be constructed which limit the number of actionable options. Imagine a nurse receives an order to start a patient on two liters oxygen via nasal cannula There isn’t a flow meter in the room so she goes to the supply room to obtain one and returns to the patient’s room. While setting up the meter, the nurse is distracted by the concerned patient’s family asking a number of appropriate questions. Because of this distraction, the nurse mistakenly connects the oxygen regulator to the air receptacle that dispenses air. In practice, there have been many reports of similar misconnects. In an attempt to prevent these types of inadvertent misconnects, the Diameter Index Safety System (DISS) was created. It creates different pin configurations for each type of regulator, making it physically impossible to insert an oxygen flow meter into any other type of port. This is an example of a physical constraint. We use physical constraints in all aspects of our life. When getting home after a tough day, you are able to unlock the door easily because the key is designed so that there is only one way to insert it into the lock. In an emergency situation, when preparing to use a cardiac defibrillator, two separate buttons must be pressed by the operator for activation to prevent accidental activation by a distracted operator in a chaotic situation.

Think about your clinical environment and identify physical constraints that assist in providing safe, consistent care: anti-tippers on wheelchairs that automatically lock the wheels whenever the patient stands or sits, sharps containers that are designed so used needles cannot be retrieved or touched after deposited. What others can you identify? One important concept to keep in mind is that effective devices should be passive, requiring no action by the user. One caution worth mentioning is that the range of options should not be restricted to the point that it is difficult to accomplish all tasks that need to get done. In this situation, people will create workarounds. When was the last time you saw a sidewalk that was designed in such a way that it guided people to walk out of their way to stay on the path? If you notice, people will create their own path by wearing out the grass to make a more efficient way to the desired endpoint. Creative architects sometimes leave sidewalks out and allow the foot paths tell them where the sidewalks should be configured. Or they come back and add to the sidewalks that already exist. On your clinical unit, how often have you seen providers fail to use safety needles because they were not passive and required the provider to do something to activate them?

In this second installment of this series focused on improving reliability in healthcare through the application of the principles of human factors engineering, we have introduced the concepts of good perceivable affordances and the introduction of physical constraints to help providers do the right thing. In the third article in this series, we will continue to explore the concept of constraints and introduce semantic, cultural, and logical constraints.

Brian Fillipo is the vice president for medical affairs at Bon Secours St. Mary’s Hospital in Richmond, Virginia. His email address is:
Sherri Barnhill is the safety and quality coordinator for patient services at Yale-New Haven Hospital in New Haven, Connecticut. Her email address is:

Gibson, J. J. (1977). The theory of affordances. In R.Shaw & J. Bransford (Eds.)., Perceiving, Acting, and Knowing. Mahwah, NJ: Erlbaum.

Norman, D. A. (1990). The design of everyday things. New York: Doubleday.