What does "Significance Level" mean?

Definition of Significance Level in the context of A/B testing (online controlled experiments).

What is a Significance Level?

Aliases: observed significance

The term significance level is often used in A/B testing to denote the observed statistical significance (post-hoc) and it is usually communicated not as the p-value (as is common in the sciences) but as the confidence level: the inverse of the p-value (1 - p) as percentage which is mathematically valid due to the duality between p-values and confidence intervals. However, it can be terminologically confusing since the leap from saying the significance level of a test is 99% to saying "there is only 1% probability that the null hypothesis is false" - logically it does not follow.

More formally the significance level is the probability of a worse fit with the data at hand (x0): p(x0) = P(d(X) > d(x0); H0) which is exactly the definition of the p-value.

When properly understood, it is the confidence level of a confidence interval that would just barely cover the upper (or lower) limit of the null hypothesis. For example, if an A/B test is significant at the 99% level with a null hypothesis of δ ≤ 0 then the lower confidence limit of a 99% one-sided confidence interval will not cover 0.

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