It's Monday night at 10pm and you're frantically calling airlines and searching for plane tickets on search sites. Your need to be in the Oslo office at 12 noon to deal with an urgent matter have reached all five airlines landing in Oslo before 11am. But every single plane is booked. You've asked about the likelihood of getting on one of the planes in case a passanger doesn't show up, and you learned that the probabilities are in the low end, 30 percent, 25 percent, 15 percent, 20 percent and 25 per cent. Disappointed, you're thinking of throwing in the towel and forget plans to reach Oslo in time. The probability is just too low.
Not really. Let's for a moment assume that for each airline, the probability of getting on a plane is unbiased and independent. In this case you'll have a 73 per cent chance to get on one a plane and reach Oslo in time.
So why do we assess the possibilities so negatively?
Disjunctive event fallacy
It's the disjunctive event fallacy leading you to expect the worst. We systematically underestimate the probability of disjunctive events, often underestimating the likelihood of events occurring independently - in the very same manner we tend to overestimate the probability of conjunctive events, which is events that occur in conjunction with each other.
So when several things must happen at once, we overestimate the probability of that happening, while if only one out of many events have to happen we underestimate the probability of it (like getting to Oslo).
Real life fallacies
What does this mean in our daily-day lives? Our tendency to overestimating conjunctive events affect our ability to plan multi-stage projects, from home renovation to major construction projects, leading us to again and again run over budget and time.
We're simply too optimistic in our assessment of the cost and time frame and surprised by unexpected setback due to our tendency to underestimate disjunctive events.
Or we underestimate the probability of disjunctive events and simply miss our flight.
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