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Planning the sample size for causal advertising lift
The required sample grows as expected lift shrinks relative to baseline volatility, but the exact calculation depends on the design inputs.
| Expected relative lift | Planning implication | Inputs required before calculating |
|---|---|---|
| Larger | Required N can be lower, all else equal | Baseline outcome, variance, allocation, power, alpha, clustering and spillover |
| Moderate | Required N can be substantially larger | The same inputs, plus a decision-relevant minimum effect |
| Very small | The design may become impractical | A pre-specified precision target, opportunity-cost limit and stopping rule |
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Reference & Evidence
Source: Author's planning worksheet based on Lewis & Rao (2015), Quarterly Journal of Economics 130(4). No single set of outcome or design inputs is assumed here.
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