Pearson's Correlation Coefficient
Pearson's product-moment correlation coefficient r = Σ[(x - x̄)(y - ȳ)] / √[Σ(x - x̄)² × Σ(y - ȳ)²]. Values range from -1 (perfect negative association) through 0 (no linear association) to +1 (perfect positive association).
Real World
Researchers studying Spotify found a strong positive Pearson's r between playlist length and listening session duration, but this did not prove that longer playlists cause longer sessions.
Exam Focus
Never say correlation 'proves' causation — always state 'association' and note a third variable or coincidence may explain the relationship.
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