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The Wilcoxon signed-rank test ranks the absolute paired differences, then compares the total ranks of the positive and negative differences. Zero differences are discarded, reducing the effective sample size.
Test statistic
W = min(W₊, W₋) over the ranks of |differences|
Normal approximation
z = (W − n(n+1)/4) / √(n(n+1)(2n+1)/24)
Six of the seven differences are negative, so W₊ is small and the test suggests a real reduction.
The sign test uses only the direction of each difference; Wilcoxon also uses their relative magnitudes, giving more power.