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A two-tailed test checks whether something has simply changed — in either direction. The significance level is split between the two tails, so each tail gets half, and the alternative hypothesis uses ≠.
What you'll be able to do
Use a two-tailed test when the question asks whether a value has or is — without specifying up or down. The alternative hypothesis uses .
Because either tail counts as significant, the significance level is between the two tails. A 5% two-tailed test uses 2.5% in each tail.
Tip — Two-tailed at 5% means comparing each tail probability to 2.5%, not 5%.
Decide which tail the observed value is in, then compare that tail’s probability to . If it is smaller, reject . Conclude in context, stating there is evidence of a change.
Formula recap
Common mistakes to avoid
Key takeaways
Test yourself
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Real past-paper questions on Two-Tailed Tests, marked mark-by-mark. How you do feeds straight into your weak-topic list, so your revision keeps targeting what actually needs work.
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