Geo Experiments Prove Incremental CTV Value for Language Learning App

A pre-test, treatment, and cooldown design measured +4.5% total sign-up lift and set a repeatable approach for upper-funnel spend.

Geo-based Test
Data Audit
Author

Diego Usai

Published

Mar 2026

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The Challenge

The client, a US-based language learning app expanding across Europe, had built a strong performance engine but struggled to justify investment in upper-funnel activity such as Video and CTV.

Platform reports showed reach, view-through, and attributed conversions, yet the commercial team could not confidently link that spend to incremental sign-ups, especially with seasonality and ongoing performance marketing in the mix.

The client needed a reproducible way to prove the true incremental value of Video and CTV and reduce budget waste as investment scaled.

The Solution

We ran geo experiments across the client’s three largest European markets, increasing CTV weight in treatment regions while keeping control regions business-as-usual.

The design used three windows: a 20-week pre-test period to establish baseline behaviour, an 8-week intervention period where spend changed, and a 6-week cooldown period to capture the delayed conversion response that often follows video exposure.

We also ensured that major confounders, including pricing, promo cadence, and ongoing performance marketing, remained stable so the readout reflected the effect of increased CTV. Success was measured through incremental paid sign-ups and first-month revenue at country level.

The Impact

Treatment regions delivered +3.1% incremental sign-ups versus control, with a further +1.4% during the cooldown period, bringing total measured lift to +4.5% across the full test readout. That matched how the category typically converts after video exposure.

Incremental CPA came out at €24, versus a blended paid-search CPA of €19 in the same period. That gap was expected, because CTV was not competing on last-click efficiency. It was creating incremental demand that later converted through other channels.

The client kept CTV in the plan, but with clearer deployment rules, using geo-based lift rather than platform reporting as the proof point and scheduling repeat tests each quarter as creative and inventory changed.

Key Takeaways

  • Upper-funnel Video and CTV are hard to value using platform reports alone – Impact is often delayed and diffused across channels.
  • Geo experiments can prove true incremental value using first-party data – Especially for upper-funnel activity with longer pay-off windows.
  • A pre-test, intervention, and cooldown structure improves interpretability – It captures both immediate and delayed conversion response.
  • The real output is a decision rule – Where spend creates incremental sign-ups, how far it scales, and where marginal returns flatten.

Tools and Techniques