A prediction or forecast is a statement about the way things will happen in the future, often but not always based on experience or knowledge. While there is much overlap between prediction and forecast, a prediction may be a statement that some outcome is expected, while a forecast may cover a range of possible outcomes.
Although guaranteed information about the information is in many cases impossible, prediction is necessary to allow plans to be made about possible developments; Howard H. Stevenson writes that prediction in business "... is at least two things: Important and hard."[1]
Prediction is closely related to uncertainty. Reference class forecasting was developed to eliminate or reduce uncertainty in prediction.[2]
The etymology of prediction is Latin (præ-, "before," and dicere, "to say").
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Outside the rigorous context of science, prediction is often confused with informed guess or opinion.
A prediction of this kind might be (inductively) valid if the predictor is a knowledgeable person in the field and is employing sound reasoning and accurate data. Large corporations invest heavily in this kind of activity to help focus attention on possible events, risks and business opportunities, using futurists. Such work brings together all available past and current data, as a basis to develop reasonable expectations about the future.
In politics it is common to attempt to predict the outcome of elections via political forecasting techniques (or assess the popularity of politicians) through the use of opinion polls. Prediction games have been used by many corporations and governments to learn about the most likely outcome of future events.
In statistics, prediction is a part of statistical inference. One particular approach to such inference is known as predictive inference, but the prediction can be undertaken within any of the several appoaches to statistical inference. Indeed, one description of statistics is that it provides a means of transferring knowledge about a sample of a population to the whole population, and to other related populations. When information is transferred across time, often to specific points in time, the process is known as forecasting.
Main article: Prophecy
Predictions have often been made, from antiquity until the present, by using paranormal or supernatural means such as prophecy or by observing omens. Methods including water divining, astrology, numerology, fortune telling, interpretation of dreams, and many other forms of divination, have been used for millennia to attempt to predict the future. These means of prediction have not been substantiated by controlled experiments, and are disputed by most, including scientists and skeptics.
In science a prediction is a rigorous, often quantitative, statement, forecasting what will happen under specific conditions; for example, if an apple falls from a tree it will be attracted towards the center of the earth by gravity with a specified and constant acceleration. The scientific method is built on testing assertions that are logical consequences of scientific theories. This is done through repeatable experiments or observational studies.
A scientific theory whose assertions are contradicted by observations and evidence will be rejected. Notions that make no testable predictions are usually considered not to be part of science (protoscience or nescience) until testable predictions can be made.
New theories that generate many new predictions can more easily be supported or falsified (see predictive power).
In some cases the probability of an outcome, rather than a specific outcome, can be predicted, for example in much of quantum physics.
Mathematical equations and models, and computer models, are frequently used to describe the past and future behaviour of something.
In microprocessors, branch prediction permits avoidance of pipeline emptying at branch instructions. In engineering, possible failure modes are predicted and avoided by correcting the mechanism causing the failure.
Accurate prediction and forecasting are very difficult in some areas, such as software reliability, natural disasters, pandemics, demography, population dynamics and meteorology.
Established science makes useful predictions which are considered to be extremely reliable and accurate; for example, eclipses are routinely predicted.
New theories make predictions which allow them to be falsified if the predictions are not borne out. For example in the early twentieth century the scientific consensus was that there was an absolute frame of reference, given the name luminiferous ether. The famous Michelson-Morley experiment ruled this out, falsifying the idea of an absolute frame and leaving the very counter-intuitive special theory of relativity as the only possibility.
Albert Einstein's theory of general relativity could not easily be tested as it did not produce any effects observable on a terrestrial scale. However, the theory predicted that large masses such as stars would bend light, in contradiction to accepted theory; this was observed in a 1919 eclipse.
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 anticipate or predict a stock market crash.
Some correlation has been seen between actual stock market movements and prediction data from large groups in surveys and prediction games.
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 (truly, estimate or compute) life expectancy.
In literature, vision and prophecy are literary devices used to present a possible timeline of future events. They can be distinguished by vision referring to what an individual sees happen. The New Testament book of Revelation (Bible) thus uses vision as a literary device in this regard. It is also prophecy or prophetic literature when it is related by an individual in a sermon or other public forum.
Divination is the attempt to gain insight into a question or situation by way of an occultic standardized process or ritual.[4] It is an integral part of witchcraft and has been used in various forms for thousands of years. Diviners ascertain their interpretations of how a querent should proceed by reading signs, events, or omens, or through alleged contact with a supernatural agency, most often describe as an angel or a god though viewed by Christians and Jews as a fallen angel or demon.[5]
Fiction (especially fantasy, forecasting and science fiction) often features instances of prediction achieved by unconventional means.