Correlation Coefficient
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Measure the linear correlation between two paired values (demo with fixed 3-point set).
Correlation ranges from −1 (perfect inverse relationship) to +1 (perfect direct relationship), with 0 meaning no linear relationship.
r = Σ((x−x̄)(y−ȳ)) ÷ √(Σ(x−x̄)² × Σ(y−ȳ)²)