Measuring long-term vs short-term impacts

There are two types of experiment results: those we can see right away and those that take weeks or months to show up. Short-term metrics like click-through rates or time-on-page provide quick feedback on immediate user reactions, while long-term metrics like retention or customer lifetime value reveal sustainable impact over time.

The challenge with measuring long-term effects is that they require extended observation periods and can be influenced by many external factors beyond your experiment. Holdout groups solve this problem by maintaining a small percentage of users on the original (control) experience even after the experiment concludes and the new version is rolled out to most users. This approach creates a clean comparison group for measuring sustained impact over months rather than days.

Some product changes show positive short-term effects but negative long-term consequences, or vice versa. For example, an aggressive promotion might boost immediate conversion while hurting long-term retention. By establishing a balanced measurement framework that tracks both immediate and delayed effects, teams can identify these trade-offs and optimize for genuine, sustainable improvement rather than temporary shifts in behavior.