Type I & Type II Errors (College Board AP® Statistics): Study Guide

Type I & Type II errors

What are the four possible conclusions of a hypothesis test?

• There are four possible conclusions of a hypothesis test

  • Two good outcomes

    • H₀ was false and H₀ got rejected

    • H₀ was true and H₀ was not rejected

  • Two bad outcomes (errors)

    • H₀ was true but H₀ was rejected (a Type I error)

    • H₀ was false yet H₀ was not rejected (a Type II error)

What is a Type I error?

• Type I errors occur when a hypothesis test gives sufficient evidence to reject H₀ despite it being true in reality

  • This is sometimes called a “false positive”

  • In a court case this would be when the defendant is found guilty despite being innocent

What is a Type II error?

• Type II errors are when a hypothesis test gives insufficient evidence to reject H₀ despite it being false in reality

  • This is sometimes called a “false negative”

  • In a court case this would be when the defendant is found innocent despite being guilty