> ## Documentation Index > Fetch the complete documentation index at: https://docs.mention-me.com/llms.txt > Use this file to discover all available pages before exploring further. # The Holdout (Control Group) > How the product referral holdout works, how to set it, and what it will let you measure. The **holdout**, also called the control group, is the share of referred visitors deliberately held back from the incentive. They arrive from a shared product link exactly as everyone else does, but no offer is rendered for them. The split happens on arrival, at the same moment the offer would otherwise be served, so the two groups are separated before either has had a chance to act. The decision is remembered, so a visitor who comes back stays in the group they started in. Comparing what the two groups go on to do is what tells you whether the incentive actually changed behaviour, rather than rewarding people who would have bought anyway. ## Why a Holdout Rather Than an A/B Test A holdout is not an A/B test arm, and A/B testing is not available on a product referral campaign. The Holdout node is the one node in the editor that is not A/B testable, for the same reason. An A/B test asks "which of these two offers is better?"; a holdout asks "is offering anything at all better than offering nothing?" [Understanding Brand and Product Referral](/knowledge-app/concepts/brand-and-product-referral#holdouts) covers the distinction in full. ## Set the Holdout The holdout is set in the **Holdout** node of the campaign editor, and again in the last step of the setup wizard when you create an incentivised campaign. The field is labelled **Holdout (control) split**, and its helper text reads "This share of visitors is held back from the incentive so you can measure the extra revenue it drives." Enter a whole number between **1** and **99**. The setup wizard defaults to **50%**. The holdout belongs to the **version**, not the campaign. Each version carries its own, so the split always matches the version that is live. Creating a new version lets you change the split without touching the one customers are currently seeing. The canvas node shows the current split as a summary pill. ## Choosing a Split There is a trade-off in both directions: * **A larger holdout** gives you a cleaner, faster read on the incentive's effect, because the control group reaches a usable size sooner. It also means fewer people see the reward. * **A smaller holdout** shows the reward to more people, but the comparison takes longer to become trustworthy, and on low-volume traffic it may never get there. Product referral traffic is narrow by nature: it is only the visitors who arrived from a share. If your product-share volume is modest, lean towards a larger holdout so the comparison has something to work with. Discuss the right split for your volumes with your Mention Me contact. ## The Split Is Locked Once Live Like every other node, the **Holdout** node is only editable on a **draft** version. Once the version is live it becomes read-only. This is deliberate. Changing the split part-way through invalidates the comparison, because the two groups would no longer have been treated consistently over the measurement period. To change it, create a new draft version, set the new split, and launch it. ## Non-Incentivised Versions Are a 100% Holdout A non-incentivised version is, in effect, a holdout of everybody: no visitor is ever shown an offer. It has no **Holdout** node, because the field only accepts 1% to 99% and so could never represent it. See [campaign modes](/knowledge-app/referral/campaigns/product-referral/campaign-modes). ## What You Can Measure Today The offer-versus-control comparison is not yet available in reporting. You can set a holdout, and visitors are correctly split between the two groups, but the platform does not yet surface the conversion and revenue comparison between them. Speak to your Mention Me contact about analysing the results in the meantime. When it does arrive, the comparison will report on the **full cohorts**, everyone who was eligible, not just the people who took the offer up. Comparing offer-takers against the control group would flatter the result, because the people most likely to redeem an offer are also the people most likely to have bought anyway. ## See Also Non-incentivised and incentivised. Where the holdout is set during setup. Where holdouts sit among the referral concepts. Change a split by launching a new version.