Simpson's paradox

The trend may reverse if you pool the observations from strata. Also related to the differences across pooled, random effects, and fixed effects Regression analysis.

• https://en.wikipedia.org/wiki/Simpson%27s_paradox

• Pasta, BMI, and Simpson’s Paradox

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

• Hernán2011simpsons: The Simpson’s paradox unraveled