Hofstadter's law is a self-referential adage, coined by Douglas Hofstadter in his book Gödel, Escher, Bach: An Eternal Golden Braid (1979) to describe the widely experienced difficulty of accurately estimating the time it will take to complete tasks of substantial complexity:[1][2]
Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.[2]
The law is often cited by programmers in discussions of techniques to improve productivity, such as The Mythical Man-Month or extreme programming.[3]