what are the odds

Notes on methods of studying and calculating coincidences... (pdf)

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Coincidences abound in everyday life. They delight, confound, and amaze us. They are disturbing and annoy- ing. Coincidences can point to new discoveries. They can alter the course of our lives; where we work and at what, whom we live with, and other basic features of daily ex- istence often seem to rest on coincidence.
Let us begin with a working definition. A coincidence is a surprising concurrence of events, perceived as mean- ingfully related, with no apparent causal connection. For example, if a few cases of a rare disease occur close to- gether in time and location, a disaster may be brewing.

The definition aims at capturing the common language meaning of coincidence. The observer's psychology enters at surprising, perceived, meaningful, and apparent. A more liberal definition is possible: a coincidence is a rare event; but this includes too much to permit careful study.

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