During return from a business trip, I was reminded of the Rule of Three decision-making concept which forms part of Shell’s Hearts and Minds safety culture toolkit. It is described in detail in this paper by Hudson et. al.

The rule of three recognizes that incidents usually have multiple causes, and human errors have an increasing role as technology reliability increases. It proposes that three “marginal” conditions - analogous to three strikes in baseball - counts as a red light or no-go decision. The paper and tool (which is available as a small .exe file) use examples for offshore transportation - including the decision to continue helicopter operations. Analysis of an incident in which 13 people lost their lives in a helicopter crash showed that the combination of environmental, crew fatigue and change factors made the incident almost inevitable - see the paper for details.

In my own example, I made a decision to get into an Uber - which turned out to be the wrong one! Applying the rule of three could have prevented this human error - in this case the car was the same make and color as the one I had ordered, but since it was new, it did not have the registration plate on the front. I was a little tired after the journey and the pickup point was busy. I was in a hurry and the first Uber driver had not shown up (a change in the plan). The Uber driver was new, so, like the example in the Rule of Three paper, getting into the wrong car was almost inevitable, with hindsight.