For what reason would a business practitioner implement a holdout group in campaign management?

Implementing a holdout group in campaign management serves the primary purpose of determining the effectiveness of campaign strategies. A holdout group acts as a control segment that is not exposed to the campaign, allowing practitioners to compare outcomes between the group that received the campaign and the group that did not. This comparison helps in evaluating key performance indicators such as response rates, conversion rates, and overall impact on business objectives. By analyzing the differences in behavior between the holdout group and those who participated in the campaign, businesses can gauge how effective their strategies are and make informed decisions for future campaigns. This approach is crucial for data-driven marketing as it provides a clearer understanding of a campaign's actual effects.

The other options do not align with the primary objective of a holdout group. Increasing participant numbers or attempting to capture all customers are not goals that a holdout group aims to achieve. Additionally, while legal compliance is important in campaign management, it is not a reason to implement a holdout group specifically; it is more focused on performance evaluation.