UNDERSTANDING HUMAN ERROR

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• What order of actions occurs when receiving money from an ATM?

Types of Error

Action Taken

Yes

Yes

No

No

Intended?

Error of Omission

Lapse

Mistake Slip

Violation: the intentional taking of a wrong action with full awareness

Slips

• Automatic and subconscious

• You have the right goal and the right intention, but take the wrong action

• Often come from skilled behaviour – why?

Why Skilled Behaviour creates Errors

• With skill, behaviours automatic

• Automatic behaviour is adaptive and efficient

• But less conscious processing allows slips

Mode Error

• A specific kind of slip

• People forget the mode the system is in

• Take the right action in the wrong mode

Mistakes

• Inappropriate goals or intentions

• Poor decisions

• Misunderstanding the situation

• Not taking all of the relevant factors into account

Why do Mistakes Happen?

• Humans use experience and heuristics to make decisions

• Not logical or rational

• Very efficient

• … but prone to error

Types of Mistakes

• Mistaken Similarity

• Misjudged Probability

• Rationalizing small events

• Social pressures, cultural factors and $

Mistaken Similarity

• Similarity to a previous situation makes people think this is the same situation

• I’ve seen this before

• Rapid conclusion without all the information

Misjudged Probability

• People estimate rare events are more likely than they are

• Gamblers

Rationalizing Small Events

• Explain away small events as normal

• A chain of small events creates a large problem

Social, Cultural $

• Pressure to take risks

• Reluctance to confront authority figures who are making mistakes

Counteracting Human Error

• Minimize or don’t use modes

• Make sure different objects look different

• Make dangerous actions hard or impossible to do

• Make actions reversible

• Make errors obvious

• Which is the best approach to take and why?

Adopting the Right Attitude

• Use mistakes to improve your design

• Ask why did it occur?

• How can I improve the design to eliminate it?

• Assume the person making the mistake was right

Design using Forcing Functions

• What is a forcing function?

• A forcing function constrains action so that mistakes can not occur

Why people work with designs that cause errors

• People tend to accept their errors and blame themselves. “I’m not smart enough, I’m a klutz”

• People will create explanations of their world, right or wrong. “I can’t run these two things at the same time.”

• When people work with your designed object they WILL develop a theory of how and why it works. “Hit it on the side.”

• YOU need to ensure that that theory is correct.