Distinguish Type I and Type II testing errors
Classify errors using both the decision and the truth of the null model.
Exam scope and assessment notes
Probability & Statistics 2 (Paper 6, A Level) · Mathematics 9709, 2026–2027, section 6.5. Paper 6 builds on Paper 5 and assumes Paper 3 calculus. Specify population hypotheses, significance level, test statistic and tail direction before inspecting the result. Calculate tail probabilities under H₀, including the observed discrete value. A p-value is not the probability that H₀ is true. Reject H₀ when the specified tail probability is at most the stated significance threshold; otherwise do not reject. The syllabus term acceptance region means the non-rejection region, not proof of H₀. For two-sided discrete examples here, use an explicitly stated equal-tail allocation; asymmetric discrete models admit different conventions, so do not treat twice the smaller tail as a universal definition. Integer critical regions can have actual Type I probability below the nominal level. Normal approximations need a suitable model and continuity correction for counts; check exact probabilities when approximation could change a close decision. Mean tests use a normal population with known variance or an adequate large independent sample with estimated variance. Small samples with unknown variance do not automatically justify the z method. Type I means rejecting a true null; Type II means not rejecting a false null at a specified alternative. Calculate error probabilities using the fixed decision region under the relevant parameter, not by changing the test after seeing data. Power is one minus Type II probability at that alternative. Quoted normal critical values and table probabilities have rounding limits. Interpret evidence in context without claiming certainty, causation or practical importance. Written practice is self-checked; cumulative assessment and human subject-teacher review remain pending.
Before you begin
- Interpret reject and do not reject.
- Recognise a specified null and alternative.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which situation is Type I error?
What you will learn
- Identify each error from its decision and truth state.
- Distinguish error probabilities from posterior claims.
Decision and truth are separate
Rejecting H₀ when it is true is Type I error. Not rejecting H₀ when it is false is Type II error. Rejecting a false null and not rejecting a true null are correct decisions. Truth may be unknown in a real application, so a reported decision does not reveal which error occurred.
For a defect-increase test from p = 0.2, rejecting when the true p is 0.2 is Type I. Not rejecting when the true p is 0.4 is Type II. The latter probability must specify the false parameter value; different alternatives give different risks.
A level does not describe every error
The significance allowance concerns rejection under H₀. It does not say that 5% of rejected claims are false or that Type II probability is 95%. These would be different conditional questions.
Power at a specified alternative is P(reject H₀ | that alternative) = 1 − β. To calculate β, retain the fixed rejection rule but change the data distribution to the given alternative parameter.
Pause and explain
What is needed to calculate Type II probability?
Worked example
Classify non-rejection of p = 0.2 when the true probability is 0.4.
Show the worked solution
- The null equality is false.
- The decision did not reject it.
- This is Type II error.
Answer Type II error.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A test rejects p = 0.2 when the true p is 0.2. Name the outcome.
Give me a hint
Use the true-null status and the reject decision.
Compare my reasoning
- The null claim is true.
- The decision is reject.
- This is a false rejection, called Type I error.
Type I error.
Look for these in your work
- State the truth condition.
- State the decision.
A test does not reject p = 0.2 when the true p is 0.4. Name the outcome.
Give me a hint
Use the false-null status and non-rejection decision.
Compare my reasoning
- The equality p = 0.2 is false.
- The decision did not reject it.
- This is Type II error at p = 0.4.
Type II error at p = 0.4.
Look for these in your work
- Identify a false null.
- State the specified alternative.
A 5% test is reported. Can β be inferred as 95%?
Give me a hint
The probabilities use different reference distributions.
Compare my reasoning
- The level constrains rejection under H₀.
- β measures non-rejection under a chosen false parameter.
- The region and alternative must be supplied to calculate it.
No; β is not automatically 1 − α.
Look for these in your work
- Separate conditioning models.
- Require an alternative parameter.
Common mistakes
- Type I and Type II refer to truth as well as decision.
- β requires a specific false parameter.
- Do not equate β with 1 − α.
What can you explain now?
Is Type II probability automatically 0.95 for a 5% test?
Compare with the explanation
No.
It depends on the fixed region and specified alternative distribution.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Type I is false rejection; Type II is missed rejection at a specified false null.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.