Extrapolation is a statistical technique aimed at inferring the unknown from the known. It attempts to predict future data by relying on historical data, such as estimating the size of a population a few years in the future on the basis of the current population size and its rate of growth.

Extrapolation may be valid where the present circumstances give no indication of any interruption in long-established past trends. However, a straight-line extrapolation (assuming a short-term trend is to continue far into the future) is fraught with risk because some unforeseeable factors almost always intervene.

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