Unlikely Optimism: The Conjunctive Events Bias

We often overestimate the likelihood of events that must happen in conjunction with one another.

We are optimistic in our estimation of the cost and schedule and surprised when something inevitably goes wrong.

• Broader categories are always more probable than their subsets. It's more likely someone has a pet than they have a cat. It's more likely someone likes coffee than they like cappuccinos. The extension rule in probability theory thus states that if B is a subset of A, B cannot be more probable than A.
• Likewise, the probability of A and B cannot be higher than the probability of A or B. It is more probable that Linda is a bank teller than that she is a bank teller and active in the feminist movement.