Individuals are not small groups, II: The ecological fallacy

tl;dr When people conclude results from group-level data will tell you about individual-level processes, they commit the ecological fallacy. This is true even of the individuals whose data contributed to those group-level results.

Individuals are not small groups, I: Simpson's paradox

tl;dr If you are under the impression group-level data and group-based data analysis will inform you about within-person processes, you would be wrong. Stick around to learn why.