What does "Theta" mean?

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

What is Theta?

The Greek small letter "theta" θ is usually used in statistics to denote an unknown parameter of interest. In A/B testing it is usually modeled as a random variable. The true value of θ is denoted θ*, while an estimator of theta, usually the maximum likelihood estimate is denoted with a hat above the letter.

The capital letter Θ is used to denote the parameter space: all possible values the variable can potentially take. It is also used with an index, e.g. Θ0 to denote the space under the null hypothesis and Θ1 to denote the space under the alternative hypothesis.

Related A/B Testing terms

Minimum Effect of Interest

Effect Size

Like this glossary entry? For an in-depth and comprehensive reading on A/B testing stats, check out the book "Statistical Methods in Online A/B Testing" by the author of this glossary, Georgi Georgiev.

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