Choose one-tailed and two-tailed alternatives before testing
Match the tail direction to the question and keep hypotheses about population parameters.
Section 6.5 · Hypothesis tests
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
- Read inequality directions.
- Distinguish p or μ from a sample result.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which alternative represents any change from mean 500?
What you will learn
- Formulate directional and non-directional alternatives.
- Distinguish parameters from observed statistics.
Write a population claim
For a claimed mean fill of 500 ml, a shortage concern uses H₀: μ = 500 and H₁: μ < 500. A question about any change uses H₁: μ ≠ 500. For a success probability, use p rather than an observed success count.
A one-tailed test puts the rejection probability in one direction. A two-tailed test allows both unusually small and unusually large results, commonly allocating half the level to each tail in the models taught here.
Choose the question before the answer
Choose direction from the study question before seeing the data. If a pre-specified increase test observes a decrease, a large lower-tail deviation does not become evidence of the specified increase.
At a fixed level, a two-tailed decision differs from a one-tailed decision. Choosing the apparently more significant direction afterwards no longer follows the pre-specified error control.
Pause and explain
What alternative represents underfilling?
Worked example
Form hypotheses for suspected underfilling relative to 500 ml.
Show the worked solution
- The parameter is population mean μ.
- H₀: μ = 500.
- H₁: μ < 500 requires a lower-tailed test.
Answer H₀: μ = 500; H₁: μ < 500.
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.
State hypotheses for a success probability suspected to exceed 0.4.
Give me a hint
Use the population probability and the requested direction.
Compare my reasoning
- The parameter is p.
- Set H₀: p = 0.4.
- Set H₁: p > 0.4; use an upper-tail rule.
H₀: p = 0.4; H₁: p > 0.4.
Look for these in your work
- Use a parameter.
- Identify the upper tail.
State hypotheses for any change from mean 100.
Give me a hint
The question includes increases and decreases.
Compare my reasoning
- The parameter is μ.
- Set H₀: μ = 100.
- Set H₁: μ ≠ 100; use two tails.
H₀: μ = 100; H₁: μ ≠ 100.
Look for these in your work
- Use μ, not x̄.
- Include both directions.
A pre-specified upper-tail test observes a low result. Can its direction be reversed while retaining the original level claim?
Give me a hint
Think about whether the new rule was chosen using the data.
Compare my reasoning
- The planned alternative was an increase.
- The observed lower result does not support that increase.
- Reversing direction afterwards changes the testing procedure.
No; report the pre-specified test and distinguish exploratory follow-up.
Look for these in your work
- Keep the planned alternative.
- Avoid a data-selected significance claim.
Common mistakes
- Hypotheses concern parameters, not x̄.
- Do not choose direction after observing the result.
- A two-tailed test uses both tails.
What can you explain now?
Can a pre-specified increase test be changed to a decrease test after seeing a low count?
Compare with the explanation
Not with its original error interpretation.
The data-dependent change alters the planned procedure.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Write parameter hypotheses and choose the direction before inspecting observations.
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.