# Campaign Categories in Holdout Experiments

Better assess the effectiveness of targeted campaigns by using a Holdout Experiment to exclude specific messages. Allow specific messages to ensure sending required content to holdout group members.

A Holdout Experiment measures the effects of excluding a group of audience members from messaging. You can compare the performance of the two audience groups in reports for selected goal events. Today's release gives you two new options when configuring your experiments:

![Configuring a Holdout Experiment](https://www.airship.com/docs/images/holdout-experiment-campaign-categories_hu_62cd312f090b1fa8.webp)

*Configuring a Holdout Experiment*

1. **Withhold by Campaign Category** — Instead of withholding all messages from holdout group members, withhold only messages with specific [Campaign Categories](https://www.airship.com/docs/reference/glossary/#campaign_categories). For example, retailers could exclude `purchase_journey` campaigns to learn how their onboarding, abandoned cart, product rating requests, and other purchase-related messages impact conversion rates.

   After setting the holdout group percentage, choose to withhold by Campaign Category, then enter a category and select its name below the entry field. Repeat for additional categories.

1. **Allow by Campaign Category** — If your experiment is set to withhold all messages, you can allow messages with specific Campaign Categories. This flexibility helps ensure your business-critical or other required messages still reach your intended audience. You can also pair it with the existing option to allow transactional messages.

   Under **Allowances**, select **Campaign Categories** and add categories as described above for withholding by Campaign Category.

When creating messages, you can set Campaign Categories per message using the dashboard or API.

## Requirements and documentation

Minimum SDKs required for [In-App Automation](https://www.airship.com/docs/reference/glossary/#iaa), [Scenes](https://www.airship.com/docs/reference/glossary/#scene), and [Surveys](https://www.airship.com/docs/reference/glossary/#survey) only: iOS 17.2.0, Android 17.2.0.

Get all the details in our [Holdout Experiments](https://www.airship.com/docs/guides/experimentation/holdout-experiments/) docs and start experimenting! 🥽 🥼 🧪