Multiple slots and interaction detection

liftstack supports testing multiple content positions (slots) within the same campaign. For example, you can test subject lines and hero blocks simultaneously.

How multi-slot testing works

Each slot is tested independently. Recipients are assigned variants for each slot using full factorial assignment, meaning all combinations are tested. If you have two variants in a subject line slot and three variants in a hero slot, all six combinations are distributed across your audience. This ensures clean attribution of results to individual content decisions.

Interaction detection (Scale tier)

When your campaign tests multiple slots, liftstack checks whether the combination matters. An interaction means that Variant A in the subject line slot performs differently when paired with Variant X vs Variant Y in the hero slot. For example, an urgency subject line might perform better when paired with an urgency hero block.

Interactions are flagged with cautious language: “We detected a possible interaction… This may warrant investigation but could also be coincidental.”

Per-slot results remain valid regardless of whether an interaction is detected. The interaction is additional context, not a change to the verdict. It helps you understand whether specific combinations are worth investigating further.

Interaction detection is available on the Scale tier.