# FORECAST - Adding a Trend Line

FORECAST functions returns linear regression estimates for up to 10 future periods based on existing data. Second parameter is a number of periods; default is 3.

Use the FORECAST function to add a trend line to an insight. The trend line uses linear regression to predict future periods based on existing data. For example, you could use a trend line to predict future sales based on historical data. Forecast inisghts must have one date/time attribute on the X axis, and no other attributes.

**Syntax**

`SELECT FORECAST(... [, number of periods])`

Optionally specify a number of periods to forecast in the future.

**Format**

number

**Default:** 3

All report filters are applied to the metric that you FORECAST except for date and time filters. To apply a date or time filter to the report, add it directly to the metric. For example:

`SELECT FORECAST((SELECT COUNT(Activity) WHERE Year = 2012), 6)`

**Examples**

`SELECT FORECAST(SUM(quantity) , 6)`

Suppose you create a forecast fo rthe totals of `Deal Size`

fact sliced by the `Month (Sale Closed)`

attribute. Use the following metric to compute the next three months based on all historical data:

`SELECT FORECAST(SUM(Deal Size))`

This metric predicts Deal Size for the next six months:

`SELECT FORECAST(SUM(Deal Size), 6)`

This metric predicts Deal Size for the next two months using data from the previous year:

`SELECT FORECAST((SELECT SUM(Deal Size) WHERE year = PREVIOUS), 2)`

This metric predicts Deal Size for the next two months using data from the previous and current years:

`SELECT FORECAST((SELECT SUM(Deal Size) WHERE year = PREVIOUS OR year = THIS), 2)`

This metric predicts Deal Size for the next four months using data from the last twelve months:

`SELECT FORECAST((SELECT SUM(Deal Size) WHERE month/year >= THIS-12), 4)`

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