Is There a Selection Bias in Gym Memberships?

There are other types of selection biases that I actually didn't necessarily know about before doing some background research for this. An interesting one is what they call attrition bias. People often sign up for long term commitments right? Or at least that's the business model of a lot of gyms. So if you did some survey of people who have Type 2 diabetes, then because of that correlation, you're biasing your results to people who are obese. Another interesting one is the causal bias. Is this similar to your polling bias? Yeah, it's, well- How's it different?