Paired t-test
The paired t-test compares the mean difference d̄ to zero using t = d̄ / (sₐ/√n), where sₐ is the standard deviation of differences and n is the number of pairs. It assumes differences follow a normal distribution. Degrees of freedom = n - 1.
Real World
NHS researchers used a paired t-test to measure blood pressure in the same patients before and after a new medication, pairing each patient's two readings to remove individual variation.
Exam Focus
Always state degrees of freedom (n−1) and check the normality assumption of differences, not original values.
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