The Subadditivity Effect: We judge the probability of a whole to be less than the sum of its parts.
Article: The Subadditivity Effect
What's more likely: natural disasters, or hurricanes plus floods plus wildfires?
They're the same thing. But people assign a higher probability to the specific list.
This is the subadditivity effect. We judge the whole to be less than the sum of its parts.
In project planning, people say a report takes "two weeks." But when you list the parts, such as research, outlining, writing, and editing, it adds up to three weeks.
The same happens with risk. "Health risks" gets a lower estimate than "heart disease plus cancer plus diabetes." Why? Specific instances feel vivid and real. Abstract categories don't activate concrete scenarios in our minds.
The solution is to break estimates into parts. List specifics rather than thinking abstractly.
For instance, when planning a project, break it into specific tasks, estimate each one, then add them up. You'll get a more accurate estimate than if you had estimated the whole directly.
The same applies to risk assessment. Don't ask "What's the risk of this venture?" Ask "What's the risk of each specific failure mode?" Then add them up.