# 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. Θ_{0} to denote the space under the null hypothesis and Θ_{1} to denote the space under the alternative hypothesis.

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