The gym members who chose the advanced class

Question

A gym offers an optional advanced fitness class. Members who chose to take it lost, on average, 6 kilograms more over three months than members who did not. The gym advertises the class as the reason for the extra weight loss.

Why can you not conclude the class caused the extra loss, and how would you test it properly?

Accepted Answer

People who opt in are different to begin with. Members who sign up for a demanding class are, on average, more motivated, more disciplined, and often already more committed to their fitness than those who skip it. Those same traits drive weight loss all by themselves, with or without the class. So the 6 kilogram gap blends two things: whatever the class actually does, and the fact that motivated people chose it. This is self-selection bias, a form of selection bias where participants sort themselves into treatment based on characteristics that also affect the outcome. Why the raw comparison i…

The gym members who chose the advanced class

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