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# Excel FORECAST Function

Forecast Equations

The Excel Forecast Function calculates a new y-value using the simple straight line equation:

where,

and

and the values of

x and

y are the sample means (the averages) of the known x- and the known y-values.

Related Function:

TREND function## 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.

The syntax of the function is:

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

In the spreadsheet below, the Excel Forecast function is used to predict an additional point along the straight line of best fit through a set of known x- and y-values (stored in cells F2:F7 and G2:G7).

As shown in cell F7 of the spreadsheet, the function to calculate the forecast y-value at x=7 is:

=FORECAST( 7, G2:G7, F2:F7 )

This gives the result *32.666667*.

Further examples of the Excel Forecast function can be found on the Microsoft Office website

## Forecast Function Errors

If your Forecast function gives you an error message, this is likely to be one of the following:

Common Errors

#N/A | - | 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! | - | Occurs if the variance of the supplied known_x's evaluates to zero. |

#VALUE! | - | Occurs if the supplied future value of x is non-numeric. |