> "extra variables" are naturally present in every study That's true, but it doesn't change the fact that if you ask people a second time to respond, that's an

> "extra variables" are naturally present in every study That's true, but it doesn't change the fact that if you ask people a second time to respond, that's an extra variable you created, not one that was naturally there already. It's not a good idea to create extra variables in the course of doing the study. It's true that the people who respond to the survey without any further prompting are not necessarily going to be representative of the entire population. But that doesn't mean you can "fix" that by further prompting. What it actually means is that surveys are a tool with only limited usefulness. That's just an inherent, inconvenient truth about surveys.