# Choice filters (Casacade)

Choice filters allow you to dynamically filter the options shown in a `select_one` or `select_multiple` question based on the answer to a previous question.

They are useful when you have linked dropdowns, such as showing cities based on the country selected, or filtering job titles based on department.

#### How choice filters work

To use a choice filter, follow these steps:

1. In the `survey` sheet, add a column named `choice_filter`.
2. In the `choices` sheet, add an extra column with the filtering attribute.
3. The `choice_filter` expression in the survey sheet must match the filter value from the selected answer.

The syntax for the filter is:

`filter_column = ${question_name}`

This compares a column in the choices sheet to the value selected in a previous question.

{% hint style="danger" %}
Choice filters will only work on the select\_one if its defined within [group](https://docs.ngsurvey.com/projects/import-export/xlsform/survey-sheet/groups) that defines a page with the field-list appearance. It will not work within a group that doesnt have a field-list apperance
{% endhint %}

#### Example: Country and city

**survey sheet**

| type                | name    | label            | choice\_filter       |
| ------------------- | ------- | ---------------- | -------------------- |
| select\_one country | country | Select a country |                      |
| select\_one city    | city    | Select a city    | country = ${country} |

**choices sheet**

| list\_name | name   | label         | country |
| ---------- | ------ | ------------- | ------- |
| country    | usa    | United States |         |
| country    | france | France        |         |
| city       | nyc    | New York City | usa     |
| city       | la     | Los Angeles   | usa     |
| city       | paris  | Paris         | france  |
| city       | lyon   | Lyon          | france  |

What happens:

1. The user selects a country in the first question (e.g. France).
2. The second question (Select a city) only shows cities where the `country` column in the choices sheet matches the selected country (`france` in this case).
3. So only "Paris" and "Lyon" will appear in the city dropdown.