SYS/DEF
Lift
The relative gap between B and A, (p_B − p_A) / p_A. +10% on a 2% CVR is +0.2 points, not a revolution.
Definition
Relative lift is (rate_B − rate_A) / rate_A. Absolute lift is the difference in points (p_B − p_A). Both are true; they do not substitute. A +20% relative lift on a 1% CVR is +0.2 points. On a 20% CVR, the same relative figure is +4 points. A sample-size MDE must say whether it is relative or absolute — mixing them accidentally multiplies n.
A point lift with no confidence interval is a sample mean, not a promise.
When it matters
To communicate a result, to size (MDE), to estimate the expected value of a test (lift × conversions × value). Always with the unit: relative or points, and with the CI.
It matters in SEA/CRO crossover: a landing lift that does not show up in CPA (different traffic, different attribution) is not yet an acquisition gain.
Pitfalls
Saying “+15%” without saying of what. Taking Monday morning’s lift (novelty). Comparing a user lift to a session lift. Turning a non-significant lift into a “positive trend”. Using relative lift as MDE in a formula calibrated in points.
Related terms
Related tool
The relative figure and its uncertainty range, glued together: Open lift and confidence interval.