Simpson's paradox

It describes a situataion where the observed trend reverses depending on whether we pool the data or divide into strata (Wikipedia). A simple illustrative example: Pasta, BMI, and Simpson’s Paradox

Also related to the differences across pooled, random effects, and fixed effects Regression analysis.

https://www.youtube.com/watch?v=ebEkn-BiW5k

Hernán2011simpsons: The Simpson’s paradox unraveled

Examples

In Europe, the excess death rate (vs. summer) increases as the winter average temperature goes up. However, the colder the weather is in each country, the more deaths occur: https://twitter.com/AinslieJstone/status/1599786478300590080 winter excess deaths across countries in europe|400