How do you calculate a simple lift in a promotional test?

• A. Lift = Metric after / Metric before
• B. Lift = (Metric after - Metric before) / Metric before
• C. Lift = Metric after - Metric before
• D. Lift = (Metric before - Metric after) / Metric before

Answer: (B)

Lift is about measuring how much the metric improves because of the promotion, relative to the starting level. You quantify that improvement by looking at the difference between the after and before values, then scaling it by the original, before value. So you compute (after − before) divided by before. This gives the proportional change, which is the lift. If you want a percentage, multiply by 100.

For example, if before you had 100 conversions and after you have 120, lift = (120 − 100) / 100 = 0.20, i.e., 20% lift. The other forms either don’t scale to the baseline or just give a raw difference or a simple ratio, which doesn't convey the same measure of relative improvement.