What is used as reference in testing null hypothesis?

In testing a null hypothesis, a reference distribution is used as a basis for comparison. This reference distribution is typically a theoretical distribution that represents what the data would look like if the null hypothesis were true. It is used to determine the probability of observing the data or more extreme data, assuming the null hypothesis is true. Common reference distributions used in hypothesis testing include the normal distribution, t-distribution, chi-square distribution, and F-distribution, depending on the specific test being conducted.