## Significance level and power of test

In this post we discuss several interrelated concepts: null and alternative hypotheses, type I and type II errors and their probabilities. Review the definitions of a sample space and elementary events and that of a conditional probability.

## Type I and Type II errors

Regarding the true state of nature we assume two *mutually exclusive* possibilities: the **null hypothesis** (like the suspect is guilty) and **alternative hypothesis** (the suspect is innocent). It's up to us what to call the null and what to call the alternative. However, the statistical procedures are not symmetric: it's easier to measure the probability of rejecting the null when it is true than other involved probabilities. This is why what is desirable to prove is usually designated as the alternative.

Usually in books you can see the following table.

Decision taken |
|||

Fail to reject null | Reject null | ||

State of nature |
Null is true | Correct decision | Type I error |

Null is false | Type II error | Correct decision |

This table is not good enough because there is no link to probabilities. The next video does fill in the blanks.

## Significance level and power of test

The conclusion from the video is that