Post hoc ergo propter hoc

Literally, "after this, therefore because of this".

This logical fallacy represents a mistaken assertion of causality. Two events, which may have no significant causal connection, are erroneously considered to represent a cause and its effect. Correlation does not equal causation.

Examples

 * I took 2 pills
 * My headache went away
 * Therefore, those pills cured my headache

Those pills might be responsible, but that causality can't be determined from this argument alone. This sort of confusion about cause and effect is demonstrated by the placebo effect and is a common basis for folk remedies and many basic religious beliefs.

Another example:


 * I prayed that I would find my lost keys
 * I found them behind the couch
 * Therefore, prayer helped me find my keys

There's no evidence that demonstrates a direct causal relationship between prayer and finding lost keys. Plenty of people have found their keys without prayer and many keys remain lost despite prayer. Yet, for those individuals who willingly accept arguments based on this fallacy, each occurrence represents another data point supporting the idea that prayer produces results.

A third example, popularized by the Church of the Flying Spaghetti Monster:


 * Between 1820 and 2000, the global mean temperature has increased
 * During this same time period, the pirate population has decreased from 45,000 to 17
 * Therefore, pirates prevent global warming.

Obviously, the link between pirates and global temps is specious at best.