# Incrementality Testing Guide > A practical guide to incrementality testing — geo holdouts, Meta Conversion Lift, ghost ads, and iROAS math — with sample-size guidance and budget decision frameworks. ## Why Platform ROAS Is Not Enough Attribution tells you who got credit. Incrementality tells you what marketing actually caused. After iOS 14.5, MER and blended CAC solved credit allocation — but not causality. The eBay field experiments showed brand-keyword paid search recovered ~99.5% of traffic organically with no measurable sales impact. ## Four Test Designs 1. **Platform Conversion Lift** — Meta, Google, TikTok user-level holdouts. Best for single-channel validation. 2. **Geo holdout tests** — Matched markets on/off. Best for cross-channel and offline impact. 3. **Switchback tests** — Temporal on/off alternation. Best when matched geos unavailable. 4. **Audience/intent holdouts** — Branded search and retargeting cannibalization tests. **Important:** Creative A/B tests compare variants — both groups see ads. They measure relative performance, not causal lift. ## Meta Conversion Lift Workflow 1. Write one business question before opening Ads Manager 2. Choose single-cell (one strategy) or multi-cell (compare strategies) 3. Set 10–20% holdout, 2–4 week duration on stable campaigns 4. Eliminate overlapping campaigns targeting the same audience 5. Read lift %, confidence interval, and compute iROAS ## Sample Size Plan ~5,000–15,000 users per arm for DTC accounts detecting ~20% lift at 95% confidence. Underpowered tests produce wide CIs that look directional but are inconclusive. ## Budget Decisions - High incrementality (>50%): scale with validation - Low incrementality (<25%) + high platform ROAS: cut or cap; budget against iROAS - Test first: branded search, retargeting, Performance Max, awareness video ## Related Tools - [Marketing Incrementality Calculator](/resources/tools/calculators/marketing-incrementality-calculator) - [A/B Test Significance Calculator](/resources/tools/calculators/ab-test-statistical-significance-calculator) - [MER Calculator](/resources/tools/calculators/mer-calculator) - [MER vs ROAS Comparison](/resources/comparisons/mer-vs-roas)