@typotheque great survey! A couple points.
1. In the initial test in the "tutorial" part, you're asked to change weight up and down until it's normal... I was stuck there for a sec because my normal wasn't what the system expected. Maybe have an "OK, I understand" button? 
2. Great to see you thought of data management etc, but you say people can email you if they want to withdraw their data, yet you don't store any identifying data...