The index of coincidence can be used to analyze whether two events are related. A coincidence does not prove a relationship, but related events may be expected to have a higher index of coincidence. From a statistical perspective, coincidences are inevitable and often less remarkable than they may appear intuitively. The odds that two people share a birthday, for example, reaches 50% with a group of just 23 (see the Birthday problem).
In The Psychology of the Psychic the author David Marks describes four distinct meanings of the term "coincidence". Marks suggests that coincidences occur because of "odd matches" when two events A and B are perceived to contain a similarity of some kind. For example, dreaming of a plane crash (event A) would be matched by seeing a news report of a plane crash on the next morning (event B).
Remarkable coincidences sometimes lead to claims of psychic phenomena or conspiracy theories. Some researchers (see Charles Fort and Carl Jung) have compiled thousands of accounts of coincidences and other supposedly anomalous phenomena (see synchronicity). The perception of coincidences often leads to occult or paranormal claims. It may also lead to a belief in fatalism, that events are pre-destined to happen in the exact manner of a prior plan or formula. This lends certain events an aura of inevitability.
Deepak Chopra and other proponents of ancient Vedic spiritual and other mystical teachings insist on the fact that there is absolutely no coincidence in the world. That everything that occurs can be related to a prior cause or association, no matter how vast or how minute and trivial. All is impacted by something related to it that is unseen or seen, cognized or not in the universe. He and many others worldwide also suggest that science, in particular mathematics, is rapidly moving towards this conclusion as well. Nonlocality theory of physics is just the latest example of phenomenon that seemed coincidental, but are in fact causal. The claim is that this and other science and mathematical conclusions can extend this to every aspect of existence.
It has also been suggested that coincidence is just the mind connecting two or more unique events; if the mind does not make the connection then there is no coincidence. (Douglas 2005)