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Some rules come from experience; others are formal procedures in manuals or rulebooks, or even less formal guides, such as cookbooks for food preparation. In either case, all we must do is identify the situation, select the proper rule, and then follow it. When driving, behavior follows well-learned rules. Is the light red? If so, stop the car. Wish to turn left? Signal the intention to turn and move as far left as legally permitted: slow the vehicle and wait for a safe break in traffic, all the while following the traffic rules and relevant signs and lights.
Rule-based mistakes occur in multiple ways:
• The situation is mistakenly interpreted, thereby invoking the wrong goal or plan, leading to following an inappropriate rule.
• The correct rule is invoked, but the rule itself is faulty, either because it was formulated improperly or because conditions are different than assumed by the rule or through incomplete knowledge used to determine the rule. All of these lead to knowledge-based mistakes.
• The correct rule is invoked, but the outcome is incorrectly evaluated. This error in evaluation, usually rule- or knowledge-based itself, can lead to further problems as the action cycle continues.
Example 1: In 2013, at the Kiss nightclub in Santa Maria, Brazil, pyrotechnics used by the band ignited a fire that killed over 230 people. The tragedy illustrates several mistakes. The band made a knowledge-based mistake when they used outdoor flares, which ignited the ceiling’s acoustic tiles. The band thought the flares were safe. Many people rushed into the rest rooms, mistakenly thinking they were exits: they died. Early reports suggested that the guards, unaware of the fire, at first mistakenly blocked people from leaving the building. Why? Because nightclub attendees would sometimes leave without paying for their drinks.
The mistake was in devising a rule that did not take account of emergencies. A root cause analysis would reveal that the goal was to prevent inappropriate exit but still allow the doors to be used in an emergency. One solution is doors that trigger alarms when used, deterring people trying to sneak out, but allowing exit when needed.
Excerpted from ‘The Design of Everyday Things’ by Don Norman