--- title: "Use the FORECASTLINEAR Function to Forecasting Data Linearly" slug: "forecast-linear-function" description: "Learn to build simple linear models in Pigment with the FORECAST.LINEAR function, using ordinary least squares to fit data." updated: 2025-05-28T10:26:32Z published: 2025-08-22T12:00:18Z --- > ## Documentation Index > Fetch the complete documentation index at: https://kb.pigment.com/llms.txt > Use this file to discover all available pages before exploring further. # FORECAST_LINEAR function ## Description Forecasts a variable by fitting a straight line to the data. It is a model that relates a response variable *Y* to an input variable *x* by the equation `Y=ax+b` The quantities *a* (slope) and *b* (intercept) are arguments of the regression model. The fitting is done using the *ordinary least squares method*. ## Syntax `FORECAST_LINEAR(Source Metric [, Ranking Dimension] [, Alternate Metric])` - `Source Metric` is the data source on which the linear regression is computed, and must be a metric with data points as an expression of Integer or number type. This metric must include the same dimension that is used in the `Ranking Dimension` argument. - `Ranking Dimension` is the dimension by which the regression is computed. If left undefined, the ranking dimension defaults to a Calendar Dimension from `Source Metric`. If `Source Metric` is defined on multiple Calendar Dimensions, you must define which dimension to use. If you want to use a dimension outside of time, you must define it here. - `Alternate Metric` is an optional argument that allows you to forecast `Source Metric` based on another metric. `Alternate Metric` must be another metric with the exact same dimensionality as `Source Metric` The last 2 arguments, `Ranking Dimension` and `Alternate Metric` are optional. ## Return type All the time series cells will be filled by an integer or decimal value starting from the first empty cell until the last value of the `Ranking Dimension` (as it is sorted). > [!TIP] > **Note:** If the regression is against an `Alternate Metric`, the forecast will only compute a value on non-empty X values. ## How the slope is calculated across dimensions The quantities *a* (slope) and *b* (intercept) are arguments of the regression model. The fitting is done using the *ordinary least squares method*. The slope a and intercept b are computed on all the dimensions that are not designed as the `Ranking Dimension`. This calculation will be performed on all items within the dimensions outside of the `Ranking Dimension`. It means that when performing a linear regression on time on a metric based on Month and Country, the resulting metric will have a different equation on all country items. For example, let's say you have a metric with Month, Country, and Product, and you use Month as the `Ranking Dimension` the linear regression would be performed for each item in the Country and Product dimensions. > [!CAUTION] > **Note**: If `Source Metric` has empty values, they won't be taken into account to compute a and b. > [!TIP] > **Note:**If the `Source Metric` has only one data point, the linear regression will return a constant function equal to the only available data point ## Examples Metric `Sales` defined on 1 Dimension | Month | Nov | Dec | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | | Sales | 1 | 3 | 5 | 4 | 9 | 13 | 16 | 17 | | | | | | | Forecasted Sales =`FORECAST_LINEAR('Sales','Month’)` ![](https://cdn.document360.io/e47cfe35-dc28-40c7-a083-6cf003073d8e/Images/Documentation/4fc7f712-bd1b-483a-9396-59c9f4c4c17f.png) Metric `Sales` defined on 2 Dimensions | Month | Nov | Dec | Jan | Feb | Mar | Apr | May | Jun | Nov | Dec | Jan | Feb | Mar | Apr | May | Jun | | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | | Country | FR | FR | FR | FR | FR | FR | FR | FR | US | US | US | US | US | US | US | US | | Sales | 1 | 3 | 5 | 4 | 9 | 13 | 16 | 17 | 1 | -1 | -3 | -5 | -4 | -9 | -13 | -16 | Forecasted Sales =`FORECAST_LINEAR('Sales','Month’)` aggregated on Countries ![](https://cdn.document360.io/e47cfe35-dc28-40c7-a083-6cf003073d8e/Images/Documentation/a8ec7685-f117-44ce-b8f5-f4599461fc3f.png) Forecasted Sales =`FORECAST_LINEAR('Sales','Month’)` not aggregated on Countries ![](https://cdn.document360.io/e47cfe35-dc28-40c7-a083-6cf003073d8e/Images/Documentation/f9c684e3-31aa-4ce2-953f-619088ebccc0.png) Metric `Cost of sales` against Metric `Sales`. | Sales | 1 | 3 | 5 | 4 | 9 | 13 | 16 | 17 | 20 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | | Cost of sales | -2.5 | -1.5 | -0.5 | -1 | 1.5 | 3.5 | 5 | 5.5 | 7 | | | | | | | | Forecasted Salary =`FORECAST_LINEAR('Cost of sales actuals’,'Month','Sales per month')` ![](https://cdn.document360.io/e47cfe35-dc28-40c7-a083-6cf003073d8e/Images/Documentation/1bc848b5-a3f8-4c32-90b0-ae02f8d2afb6.png) ## References [https://www.sciencedirect.com/topics/mathematics/simple-linear-regression](https://www.sciencedirect.com/topics/mathematics/simple-linear-regression) > [!TIP] > **More of a hands-on learner?** > > Talk to your Customer Success Manager about downloading the Functions and Modifiers in Pigment Application into your workspace. It includes examples of every formula and modifier in Pigment!