# Creating Aggregations ## Aggregation in the Variable wizard Aggregations are created in the [**Variable wizard**](/variables/variable-basics/variable-setup.md).

Screenshots of the Variable wizard, showing how to create an Aggregation — Creating an Aggregation in the Variable dialog

To create an Aggregation you will need to perform the following steps, as per the screenshots above: * Select the `Aggregation` **Variable type** **(1)** * Assign at least one [**Dimension**](/dimensions/introduction-to-dimensions.md) to the Variable **(2)** * Set the [**Unit**](/units/introduction-to-units.md) for the Variable, in the **Unit step** * In the **Aggregated Variables step**, select the Variables to be aggregated **(3)**. The **Item Statuses** table **(4)** shows which items have been selected, and can be used to filter the Variable list to matching items. {% hint style="warning" %} Selected Variables must have the correct Unit so that Models can match them correctly to the target items. The below section explains several ways to set up these Variables. {% endhint %} ## Creating Variables with the correct Unit for Aggregation Each aggregated Variable must have a Unit which has been modified to match one of the child items of the Aggregation Variable. For instance, in the above example, the Aggregation Variable (`All costs by cost type`) has a Unit of `GDP` and a Dimension of `Cost type`, which has seven children (`Central staff`, `Gym staff`, etc.). To match a `Central staff` Variable, we need a Variable with a Unit of `GDP` which has had a `Central staff` modifier applied. To match a `Gym staff` Variable, we need a Variable with a Unit of `GDP` which has had a `Gym staff` modifier applied, and so on for each item. The screenshot below shows such Units for the `Central staff` and `Gym staff` items.

Variables with modified Units for Aggregation

### Manual creation These Variables can be created manually. In this case, the Unit should be specified with appropriate modifiers added, in the **Unit step** of the Variable wizard, as demonstrated in the gif below.

GIF showing the user setting a context modifier on a unit part in the Unit page of the Variables wizard — Setting a Unit Modifier

Read [**here**](/units/key-unit-concepts.md#context) to understand more about Unit Modifiers. ### Automatic creation If you have not already created the Variables to be aggregated, you can automatically create them in the [**Aggregated Variables step**](#aggregation-in-the-variable-dialog) of the Variable dialog. To do so, perform the following steps, shown in the example screenshots below: * Navigate to the **Aggregated Variables step** * Select any items that you want to create in the **Item statuses** **(1)**. By default, all items will be selected. Selected items are indicated by the filter

icon. * Press the **Create selected** button **(2)** * A Variable for each item will be automatically created, each with a correctly modified Unit **(3)** * These new Variables should then be selected and edited as desired

Example of automatic aggregated Variable creation

### Aggregating Variables with Dimensions You can aggregate Variables which themselves have Dimensions, if some or all of the child items of their Dimension are present in the Aggregation Variable. The screenshot below shows an example of this feature. In this case, the `All costs by cost type` Variable references a Variable **(1)** called `Costs: Recurring`, which has the `Simple recurring cost type` Dimension applied. The `Simple recurring cost type` Dimension has four cost items as children: `Marketing cost`, `Rent`, `Equipment`, and `IT & Technology` **(2)**. Each of these four items are also present in the `Cost type` Dimension that is applied to `All costs by cost type`. Hence selecting `Costs: Recurring` means that Models is able to determine values for those four items, as indicated in the **Item statuses control ****(3)**. {% hint style="info" %} In this case, you do not need to apply any Modifiers to the Unit of `Costs: Recurring.` `Costs: Recurring` already has Unit `GBP`, and the relevant Modifiers are applied in the background by the `Simple recurring cost type` Dimension {% endhint %}

Example of an Aggregation referencing a Variable with a Dimension

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