Thank you for the context! With Facebook at the gate, selection bias is massive, and clearly not compensated for. With the chance of getting a 25-50€ gift cert

Thank you for the context! With Facebook at the gate, selection bias is massive, and clearly not compensated for. With the chance of getting a 25-50€ gift certificate as the motivator, many will have filled the survey very quickly just to get their chance. I would suggest conducting interviews at Buddhist or Yoga retreats instead.