Type I Error
A Type I error occurs when you reject H₀ even though H₀ is true. The probability of making a Type I error is α (the significance level). Type I errors are controlled by choosing a small significance level. They represent false positives or incorrect rejection of a true hypothesis.
Real World
In 2020 the FDA required Pfizer to prove vaccine efficacy rigorously, using α = 0.05, specifically to limit the chance of approving a vaccine that is actually ineffective (a Type I error).
Exam Focus
State Type I error clearly: 'reject H₀ when H₀ is true'; examiners penalise vague descriptions like 'getting it wrong'.
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