Spurious correlation: tea imports and life expectancy

Question

Over 40 years, a country's annual tea imports and its life expectancy both rose steadily, and their yearly levels correlate at $r = 0.97$ . A wellness blogger cites this as evidence that tea extends life.

Why is this correlation weak evidence, and how would you check for a real link?

Accepted Answer

The flaw: two upward trends correlate automatically. Both tea imports and life expectancy climbed over four decades because the country was developing: rising income lifted tea consumption and, separately, improved nutrition, sanitation, and medicine that raised life expectancy. Both are nonstationary, trending series, so they correlate strongly in levels regardless of any direct link. Development (and time itself) is a common driver behind both; this is a spurious correlation. How to test for a real relationship. 1. Use changes, not levels. Correlate year-over-year changes in tea imports wit…

Spurious correlation: tea imports and life expectancy

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