Design of Everyday Things - Norman, Don [62]
DETECTING SLIPS
Although slips are relatively easy to detect because there is a clear discrepancy between goal and result, detection can only take place if there is feedback. If the result of the action is not visible, how can a misaction be detected? Even when a mismatch is noted, the person may not believe that the error occurred. Some trail of the sequence of actions that was performed is valuable.
Even when an error has been detected, it may not be clear what the error was.
“Alice” was driving a van and noticed that the rearview mirror on the passenger side was not adjusted properly. Alice meant to say to the passenger on the right, “Please adjust the mirror,” but instead said “Please adjust the window.”
The passenger, “Sally,” was confused and asked, “What should I do? What do you want?”
Alice repeated the request: “Adjust the window for me.”
The situation continued through several frustrating cycles of conversation and attempts by the passenger to understand just what adjustments should be made to the window. The error-correction mechanism adopted by the driver was to repeat the erroneous sentence more and more loudly.
In this example, it was easy to detect that something was wrong but hard to discover what. Alice believed the problem was that she couldn’t be understood or heard. She was monitoring the wrong part of the action sequence—she had a problem of level.
Actions can be specified at many different levels. Suppose I were driving my car to the bank. At any given moment, the action being performed could be described at many different levels:
• Driving to the bank
• Turning into the parking lot
• Making a right turn
• Rotating the steering wheel clockwise
• Moving my left hand upward and to the right and my right hand downward
• Increasing the tension on the sternocostal portion of the pectoralis major muscle
All these levels are active at the same time. The most global description (the one at the top of the list), is called the high-level specification. The more detailed descriptions, the ones at the bottom of the list, are called the low-level specifications. Any one of them might be in error.
It is often possible to detect that the result of an action is not as planned, but then not to know at which level of specification the error has taken place.
Problems of level commonly thwart the correction of error. My collection of slips includes several examples in which a person detects a problem but attempts to correct it at the wrong level.
One frequent example is the nonworking key, reported to me both for cars and homes. Someone goes to his or her car and the key won’t work. The first response is to try again, perhaps holding the key more level or straight. Then the key is reversed, tried upside down. When that fails, the key is examined and perhaps another tried in its stead. Then the door is wiggled, shaken, hit. Finally, the person decides that the lock has broken, and walks around the car to try the other door, at which point it is suddenly clear that this is the wrong car.
In all the situations I have examined the error correction mechanism seems to start at the lowest possible level and slowly works its way higher. Whether this is universally true I do not know, but the hypothesis warrants further examination.
DESIGN LESSONS FROM THE STUDY OF SLIPS
Two different kinds of design lessons can be drawn, one for preventing slips before they occur and one for detecting and correcting them when they do occur. In general, the solutions follow directly from the preceding analyses. For example, mode errors are minimized by minimizing modes, or at least by making modes visible.
Cars provide a number of examples of how design relates to error.
A variety of fluids are required in the engine compartment of an automobile: engine oil, transmission oil, brake fluid, windshield washer solution, radiator coolant, battery water. Putting the wrong fluid into a reservoir could lead to serious damage or even an accident. Automobile manufacturers