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