The Opposite of Fitt’s Law, Redux
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The co-pilot mistook the rudder trim knob for the cockpit door lock switch so when he “opened the door” for his captain, he actually caused the jet to roll and drop 1,900 meters in 30 seconds. According to internal investigations, “the narrow-body aircraft continued to roll until it reached 131.7 degrees to the left, leaving it almost belly-up. Its nose pointed down as much as 35 degrees at one point.
Seems a ridiculous error until you realize:
As you can see in that picture, the rudder knob and the cockpit door lock switch are no more than 10cm apart. Unlocking the door (the small knob) and moving the rudder left (the big knob) is the same direction.
We’re reminded of this corollary to Fitt’s Law:
If we should make UI elements we want users to click on large, and ideally place them at corners or edges for maximum clickability — what should we do with UI elements we don’t want users to click on? Like, say, the “delete all my work” button?
Alan Cooper, in About Face 3, calls this the ejector seat lever:
“In the cockpit of every jet fighter is a brightly painted lever that, when pulled, fires a small rocket engine underneath the pilot’s seat, blowing the pilot, still in his seat, out of the aircraft to parachute safely to earth. Ejector seat levers can only be used once, and their consequences are significant and irreversible.Applications must have ejector seat levers so that users can—occasionally—move persistent objects in the interface, or dramatically (sometimes irreversibly) alter the function or behavior of the application. The one thing that must never happen is accidental deployment of the ejector seat.”
Also recently spotted, but of lesser potential consequence, is this red (touch me!) emergency button at kid-level:
The next time you’re constructing a user interface, you should absolutely follow Fitts’ law. It just makes sense. But don’t forget to follow the opposite of Fitts’ law, too — uncommon or dangerous UI items should be difficult to click on!
