A fudge factor is an element inserted into a calculation, formula or model to allow for error or uncertainty. Fudge factors are also used to make something congruent with an observed or desired result. The word fudge, in this context, means to be intentionally unclear or imprecise.
Fudge factors are commonly used to include variables in a calculation when a precise quantity is unknown or when a variable is subject to unpredictable change. In project management, fudge factors are commonly used in time estimates. In calculating the duration of some phase of the project, for example, a project manager might add a fudge factor of 50 percent to the estimate of how long that phase should take. The addition is intended to make it possible to meet the deadline even if unforeseen events cause delays, as they often do.
The extra allotment can also cause problems, however. Because people tend to use all available time for a deadline, a project phase that might have been completed in two weeks often ends up taking three. To avoid that situation, it's wise to be conservative in the use of fudge factors.
Fudge factors are commonly used in the sciences to make a calculation or formula work, when it's considered that the other elements involved are correct. If an empirically observed phenomenon cannot be explained by a theory, for example, a fudge factor may be introduced to make the theory work. Despite the requirement that scientific research be objective, cognitive bias -- particularly confirmation bias -- can pose a risk to the validity of a study through the introduction of fudge factors.
One of the most famous fudge factors of all time is Einstein's cosmological constant. The scientist added the constant to his general theory of relativity to make it conform to his belief that the universe was neither contracting or expanding but was static. The fudge factor provided a repulsive force that explained why the universe did not collapse upon itself. When it was discovered that the universe is in fact expanding, however, Einstein had to remove the cosmological constant from his theory.