I recently heard a story that reminded me of how people often struggle to manage risk and reliability.
A group of about 20 high school students were travelling across the U.S. by air for a school-sponsored activity. The students had been split between two different flights, with 10 students on each flight. Since there was only one adult in the group, the 10 youngest students were put on the flight with the adult, because the younger students seemed more at risk if their flight were delayed or diverted.
As it happened, the flight for the older students was cancelled, so those students were left to fend for themselves until the next available flight the next day.
Someone who is concerned with risk and reliability will ask, why were the students split into two groups, resulting in half of the students being stranded without an adult? Did the airline only have enough seats for half of the group on each flight? It was suggested to me that, as a matter of school policy, large groups were often split in two, so that if one of the planes crashed, then the number of students lost in the tragedy would be reduced.
Would such a policy make sense from a risk management perspective? The following Cause Map shows the basic root cause analysis for losing all of the students in a plane crash, and how separating the students into two groups prevents (that is, reduces the risk of) losing all of the students.
However, splitting the students into two groups greatly increases the possibility of some students being stranded without an adult, as shown in the Cause Map below.
The missing piece of this risk management puzzle is the probabilities that the causes in the Cause Maps occur. Risk can be quantified by multiplying the consequence of a failure times its probability. Over the past year, 1.63% of U.S. plane flights have been cancelled, so if the students are split into two groups, the unescorted group will become stranded at least 1 time in 100. On the other hand, the probability that a plane flight will crash is on the order of 1 in 10,000,000 and, contrary to what you may think, it is rare that more than a quarter of the passengers die in a crash.
Creating a policy to split up the students is a possible solution to the problem of losing all students in a single crash, but now that the risks are better understood, is it a very effective solution for reducing the overall risk to the students?
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