Prediction

The etymology of this word is Latin (from pr&aelig;- "before" plus dicere "to say").



Informal prediction (hypothesis)
Outside the rigorous context of science, prediction is often confused with informed guess or opinion. A prediction of this kind might be valid and useful if the predictor is a knowledgeable person in the field and is employing sound reasoning and accurate data.

Opinion Polls
In politics it is common to attempt to predict the outcome of elections (or assess the popularity of politicians) through the use of opinion polls.

Supernatural (prophesy)
Predictions have often been made, in pre-scientific times and still today, by resorting to paranormal or supernatural means, such as prophecy. Pseudoscience disciplines include water divining, astrology, numerology and fortune telling. So far none of these means of prediction have been proven under controlled conditions and are heavily criticised by scientists and skeptics.

Anticipatory science forecasts
In a scientific context, a prediction is a rigorous (often quantitative) statement forecasting what will happen under specific conditions, typically expressed in the form If A is true, then B will also be true. The scientific method is built on testing assertions which are logical consequences of scientific theories. This is done through repeatable experiments or observational studies.

A scientific theory whose assertions are not in accordance with observations and evidence will probably be rejected. For example, theories such as String theory make no testable predictions, and thus remain protosciences until testable predictions are known to the community.

Additionally, if new theories generate many new predictions, they are often highly valued, for they can be quickly and easily confirmed or falsified (see predictive power). In many scientific fields, desirable theories are those which predict a large number of events from relatively few underlying principles.

Quantum physics is an unusal field of science because it enables scientists to make predictions on the basis of probability.

Mathematical models and computer models are frequently used to both describe the behaviour of something, and predict its future behaviour.

In microprocessors, branch prediction permits to avoid pipeline emptying at microcode branchings. Engineering is a field that involves predicting failure and avoiding it through component or system redundancy.

Some fields of science are notorious for the difficulty of accurate prediction and forecasting, such as software reliability, natural disasters, pandemics, demography, population dynamics and meteorology.

Example of scientific hypothesis
In the 1840s the renowned Austro-Hungarian physician Ignaz Semmelweis noticed that women giving birth in the Vienna lying-in hospital were dying in one building, but surviving in another.

Upon considering the cause, he found that the surviving women were attended by midwives and not by student physicians. Thus he proposed the hypothesis that the physicians were a factor in the deaths.

This proposition impelled Semmelweis to refine the factor. What was the difference between the midwives and the doctors? After more thought, Semmelweis decided that the cadavers which the student doctors were touching must be part of the factor.

What could the doctors do to avoid the factor? Semmelweis predicted that, if the doctors were to wash their hands, then the cadaver factor will be avoided.

Semmelweis therefore instructed the student doctors to wash their hands, and the women who were attended by the doctors survived. Thus his prediction was successful, and his explanation was validated.

(Semmelweis, 1861. The Etiology, Understanding, and Prophylaxis of Childbed Fever)

Other examples abound in the history of science, ranging from expected predictions which did not occur (such as the Michelson-Morley experiment) to new and radical predictions which shockingly confirmed one theory over another (such as the bending of light around the sun seen in the 1919 eclipse, a prediction of Albert Einstein's theory of General relativity).

Finance
Mathematical models of stock market behaviour are also unreliable in predicting future behaviour. Consequently, stock investors may anticipate or predict a stock market boom, or fail to anticpate or predict a stock market crash.

An actuary uses actuarial science to assess and predict future business risk, such that the risk(s) can be mitigated.

For example, in insurance an actuary would use a life table to predict life expectancy.

Prediction in fiction
Fiction (especially fantasy, forecasting and science fiction) often features instances of prediction achieved by unconventional means.

In fantasy literature, predictions are often obtained through magic or prophecy, sometimes referring back to old traditions. For example, in J. R. R. Tolkien's The Lord of the Rings, many of the characters possess an awareness of events extending into the future, sometimes as prophecies, sometimes as more-or-less vague 'feelings'. The character Galadriel, in addition, employs a water "mirror" to show images, sometimes of possible future events.

In some of Philip K. Dick's stories, mutant humans called precogs can foresee the future (ranging from days to years). In the story called The Golden Man, an exceptional mutant can predict the future to an indefinite range (presumably up to his death), and thus becomes completely non-human, an animal that follows the predicted paths automatically.

In the Foundation series by Isaac Asimov, a mathematician finds out that historical events (up to some detail) can be theoretically modelled using equations, and then spends years trying to put the theory in practice. The new science of psychohistory founded upon his success can simulate history and extrapolate the present into the future.

In Frank Herbert's sequels to Dune, his characters are dealing with the repercussions of being able to see the possible futures and select amongst them. Herbert sees this as a trap of stagnation, and his characters follow a Golden Path out of the trap.

In Ursula K. LeGuin's The Left Hand of Darkness, the humanoid inhabitants of planet Gethen have mastered the art of prophecy and routinely produce data on past, present or future events on request. In this story, this was a minor plot device.