Treating correlation as causation without ruling out reverse causation or a common cause.
Ice cream sales and drowning both rise in summer. The harder version is reverse causation: exercise and health correlate, and the causation runs both ways.
In practice
- Firms with a wellbeing programme have lower absence, and healthier firms are also the ones that can afford the programme.
- Students who attend revision classes score higher, and the students who attend are the ones who were going to revise anyway.