Post hoc ergo propter hoc

Post hoc ergo propter hoc (Latin: 'after this, therefore because of this') is an informal fallacy which one commits when one reasons, "Since event Y followed event X, event Y must have been caused by event X." It is a fallacy in which an event is presumed to have been caused by a closely preceding event merely on the grounds of temporal succession. This type of reasoning is fallacious because mere temporal succession does not establish a causal connection. It is often shortened simply to post hoc fallacy. A logical fallacy of the questionable cause variety, it is subtly different from the fallacy cum hoc ergo propter hoc ('with this, therefore because of this'), in which two events occur simultaneously or the chronological ordering is insignificant or unknown. Post hoc is a logical fallacy in which one event seems to be the cause of a later event because it occurred earlier.

Post hoc is a particularly tempting error because correlation sometimes appears to suggest causality. The fallacy lies in a conclusion based solely on the order of events, rather than taking into account other factors potentially responsible for the result that might rule out the connection.

Post hoc ergo propter hoc is an easy fallacy to detect when it is blatant, but even the best of scientists and statesmen are occasionally misled by it.

Pattern
The form of the post hoc fallacy is expressed as follows:
 * A occurred, then B occurred.
 * Therefore, A caused B.

When B is undesirable, this pattern is often combined with the formal fallacy of denying the antecedent, assuming the logical inverse holds: believing that avoiding A will prevent B.

Examples

 * A tenant moves into an apartment and the building's furnace develops a fault. The manager blames the tenant's arrival for the malfunction. One event merely followed the other, in the absence of causality.
 * Brazilian footballer Pelé blamed a dip in his playing performance on having given a fan a specific playing shirt. His play recovered after receiving from a friend what he was told was the shirt in question, despite it actually being the same shirt he'd worn during his poor performance.
 * Reporting of coincidental vaccine adverse events, where people have a health complaint after being vaccinated and assume it was caused by the vaccination.