Basic Description

The Excel Forecast function predicts a future point on a linear trend line fitted to a supplied set of x- and y- values.

Syntax: FORECAST( x, known_y’s, known_x’s )

Where the arguments are as follows :

x A numeric x-value for which you want to forecast a new y-value
known_y’s An array of known y-values
known_x’s An array of known x-values.

Note that the length of the known_x’s array should be the same length as known_y’s, and the variance of the known_x’s must not be zero.


Forecast Function Example

The spreadsheet below shows an example of the Forecast function used to predict an additional point along the straight line of best fit through the known_x’s and known_y’s in cells F2:F7 and G2:G7. These known points are shown in the graph on the same spreadsheet.

Example of use of the Excel Forecast Function

The above example gives the result 32.666667.


Forecast Function Errors

If your Forecast function gives you an error message, use the table below to look up the likely cause:

#VALUE! Produced if the supplied future x value is non-numeric
#NA Occurs if there are problems with the supplied known_x’s or known_y’s arrays – i.e. if either of these arrays is empty, or if the arrays are of different lengths.
#DIV/0! Produced if the variance of the supplied known_x’s evaluates to zero.

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