Statistical Significance: When to Call It

You run an experiment for 5 days. The variant is up 8%. You ship it.

Two weeks later, the metric is back to where it started. What happened?

You got fooled by noise. The 8% lift was random fluctuation, not a real effect. You saw a pattern in randomness and acted on it. This happens constantly, and it's the number one reason teams lose trust in experiments.

Statistical significance is the tool that prevents this. It tells you: "Given the data you have, how confident should you be that this result is real and not random noise?"

1 / 10

Ship it

Statistical analysis notebook that determines when an experiment has reached significance

Feature Flags and Progressive Rollouts

Multi-Armed Bandits and Adaptive Experiments

Rex — Statistical Significance: When to Call It

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Ask Rex anything about this lesson

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Experiments & Decision Systems/Experiment Design/Statistical Significance: When to Call It

30 minLesson 3 of 6

Statistical Significance: When to Call It

You run an experiment for 5 days. The variant is up 8%. You ship it.

Two weeks later, the metric is back to where it started. What happened?

Statistical significance is the tool that prevents this. It tells you: "Given the data you have, how confident should you be that this result is real and not random noise?"

1 / 10

Ship it

Statistical analysis notebook that determines when an experiment has reached significance

Feature Flags and Progressive Rollouts

Multi-Armed Bandits and Adaptive Experiments

Experiments & Decision Systems/Experiment Design/Statistical Significance: When to Call It

30 minLesson 3 of 6

Statistical Significance: When to Call It

You run an experiment for 5 days. The variant is up 8%. You ship it.

Two weeks later, the metric is back to where it started. What happened?

Statistical significance is the tool that prevents this. It tells you: "Given the data you have, how confident should you be that this result is real and not random noise?"

1 / 10

Ship it

Statistical analysis notebook that determines when an experiment has reached significance

Feature Flags and Progressive Rollouts

Multi-Armed Bandits and Adaptive Experiments

Rex — Statistical Significance: When to Call It

Gemini Flash

Ask Rex anything about this lesson

Get help with concepts, run prompts, practice exercises, or ask what to do next.

Experiments & Decision Systems/Experiment Design/Statistical Significance: When to Call It

30 minLesson 3 of 6

Statistical Significance: When to Call It

You run an experiment for 5 days. The variant is up 8%. You ship it.

Two weeks later, the metric is back to where it started. What happened?

Statistical significance is the tool that prevents this. It tells you: "Given the data you have, how confident should you be that this result is real and not random noise?"

1 / 10

Ship it

Statistical analysis notebook that determines when an experiment has reached significance

Feature Flags and Progressive Rollouts

Multi-Armed Bandits and Adaptive Experiments