Use STAR: Situation, Task, Action, Result with a quantified impact.
How to answer it
The interviewer is checking that you can change what someone else did, not that you ran an analysis. So the story needs a person who believed something, a piece of evidence you built, and a decision that came out differently.
STAR, with the emphasis on the last two letters:
Situation: what was about to happen and who was driving it. "The product lead planned to cut the free tier's mock interviews from 5 to 2 to push upgrades."
Task: what you set out to establish. "Whether free users who ran more mocks were more or less likely to upgrade."
Action: what you actually did, specifically. "Pulled 6 months of cohorts, found upgrade rate rose with mocks used up to 3 and flattened after. Showed that the proposed cut would remove the point where most upgrades happened. Suggested 3 instead of 2 and an upgrade prompt at the third mock."
Result: the decision and the number. "They went with 3 plus the prompt. Upgrade rate rose 18% over the next quarter instead of the drop the model of 2 predicted."
Then the reflection interviewers reward: what made it persuasive was not the chart but framing it in the lead's own goal, upgrades, and offering an alternative rather than a veto.
Pick a story where the outcome was checkable afterwards. "They agreed with me" is weaker than "and here is what happened next".
What they are checking: that you changed a decision, quantified it, and can say why your argument landed.
Common mistake: describing an analysis in detail and ending with "and I presented it to the team".