Glossary
Subadditivity Effect
What is the subadditivity effect?
The subadditivity effect occurs when the judged probability of a category is lower than the sum of the judged probabilities of its separate parts.
A numerical example
Suppose each record is assigned exactly one category: hurricane, flood, wildfire, or other. Someone estimates a 20% probability for the first three categories combined, but gives hurricane, flood, and wildfire 10% each when asked separately. The parts add to 30%, above the original 20% judgment.
The categories must not overlap for this comparison. Real disasters can involve both a hurricane and flooding, so simply adding their probabilities could double-count events.
Why unpacking matters
Breaking a category into parts can draw attention to possibilities that the broader label brings to mind less readily. Rottenstreich and Tversky examine how descriptions affect these probability judgments.
Applications
In a risk assessment, keep the population, time period, and event definitions constant. Check for overlap before treating inconsistent totals as evidence of subadditivity.
Why it matters
The description of an event can affect the estimate people give it. A clean comparison needs careful arithmetic and equally careful definitions.