Using linear and quadratic models for forecasting

Having obtained the equation of a trend line by least squares, forecasting is simply a matter of inserting future time values into the equation.

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Dangers in forecasting

It is important to realise that the forecasts from linear or quadratic models are highly dependent on the type of line or curve that is chosen for modelling. The dangers are the same as those for extrapolation in bivariate relationships.

Beware forecasting many time periods into the future — the shape of the actual trend line might be different from your model.