A coincidence is a random co-occurrence of two or more events that are perceived to be meaningfully associated with each other, even though there is no meaningful causal relationship linking them. A collision between an ambulance carrying an injured bullfighter and a cattle truck would constitute a coincidence, while internal bleeding following ingestion of broken glass would not. The need to distinguish true associations from coincidences is critical to good medical decision making, yet the human mind is ill equipped to make this distinction.

Co-occurrences of events can be perceived as meaningful when they happen along a number of dimensions, such as time (e.g., when a patient develops symptoms shortly after taking a drug), space (e.g., when multiple cases of a rare disease occur in the ...