# Build an expression You build an expression on the **Expression** tab of the [data picker](), wherever you need it: in a filter, a measure, a data grid column, or a personalised message. For what expressions are and the rules they follow, see [Expressions](). ## Before you begin Check that you can use expressions, and that you have what you need: - **Licence**: The **Expression** tab isn't available with a Selector licence. Expressions in message personalisation need the [Analyser bundle or higher](). - **Variable names**: Use the exact names of variables in your FastStats system, in square brackets, such as `[Surname]`. A name that doesn't match gives an error. - **Knowledge**: You'll need to know how to write Apteco expressions. See the [FastStats expression reference](), or start from an [example expression](). The [rules for expressions]() explain what Orbit doesn't support. ## Write an expression The steps are the same wherever you open the data picker. To write an expression: 1. Open the data picker where you want to use the expression. For example, select **\+ Add Filter**, **\+ Add Measure**, or **\+ Add Column**. In message personalisation, add an **Expression** content field, then select **\+ Add Expression**. 2. Select the **Expression** tab. If there's no **Expression** tab, you can't use an expression in this place, or your licence doesn't include expressions. 3. Enter a **Name** for the expression. Orbit uses it to identify the expression wherever you use it. 4. Choose the **Table** to calculate a value for. The expression works out one value for each record of this table, and can use variables from it or from any table above it. 5. Write the **Expression**. For example, to work out each supporter's age: Expression ```apteco AgeYears([DOB]) ``` 6. Check the message under the expression. Orbit checks the expression as you type. When it's valid, Orbit shows **This expression is valid**. When it isn't, Orbit says what's wrong, so you can correct it. 7. For a text expression in a data grid column, set the **String Length (Characters)**. The default is 20 characters, and the maximum is 32,767. 8. Select **Apply**. > [!TIP] > > Test an expression on a small audience or a single tile before you rely on it, to check it returns the values you expect. ## What happens next What you do after you select **Apply** depends on where you're using the expression: - **In a filter**: Orbit opens the same dialogue as for a variable of the expression's output type. For a numeric expression, you set [ranges of values](), such as more than 20. For a text expression, you choose how to [match text](), such as **Is** or **Contains**, with **Match case**. For a date expression, you set a [date range](). - **As a measure**: Choose a **Function** to summarise the values, such as **Mean** or **Sum**. The [measure functions]() explain each one. - **As a data grid column**: Give the column a title, then select **Apply** in the tile editor's side panel. - **In message personalisation**: The value becomes a column in your campaign's output file, headed with the content field's name. Once you've added it, you can't view the expression in Orbit. Find out more about [expressions in message personalisation](). > [!NOTE] > > **Example** > > To show how far supporters in each region live from your head office, add a cube tile with **Region** as its dimension. For the measure, write a numeric expression that calculates each supporter's distance in miles from the office's postcode: > > Expression > > ```apteco > UKPostcodeDistance([Postcode], "CV34 4EH") > ``` > > Then choose the **Mean** function. The cube shows the average distance for each region.