The Bias Effect

Added on Jul 21, 2015

Which line is longer? (See inset right) I would guess most of you said that both lines are the same length. We recognize this as an old optical illusion from our psychology text books, and we already know the right answer.

Wrong. That was a bias. I made the top line 10% longer.

This is the nature of jumping to conclusions. Lacking information or knowledge – our minds are fast and efficient and help us by filling-in the gap with something we already know. Biasing in judgments and decision-making comprise 34% of all human error leading to patient harm. This is from a study of 2,329 acts (where the individual failure mode was known) leading to 2,375 cases of patient harm from 2013-2014.

The best way to prevent a bias is by practicing strong questioning attitude, either alone or as a team. How do I know this to be correct? If this is important to my choice – how can we confirm this information to be correct? Or should I just stop since I am unsure?

Avoid the bias effect by knowing instead of assuming.

Questions to consider:

  • 1. Where in our practice do we sometimes make assumptions instead of knowing for a fact? Are these good assumptions or assumptions for which we would be better-off if we confirmed them as fact?

  • 2. And how effective are we in practicing questioning attitude as a team? Do we pace ourselves by diligently knowing by confirming – or do we wear-out ourselves by jumping to conclusions?



The Muller-Lyer effect is an optical illusion consisting of two arrows. Viewers are asked which of the two lines is longer. The test was developed in 1889 by Franz Muller-Lyer, a German sociologist.

Eddie Obeng first showed us this effect in his TED Talk describing how the rules changed at midnight – 15 years ago. Eddie Obeng is a British organizational theorist at the Henley Business School.