Actually, it can be private if they use your email adress... As long as they don't link the data to your email adress. In good research, the email adress could be used to verify that a person did the survey and prevent him/her from taking it again.
(Ik heb juist mijn vak "Kwalitatieve Onderzoeksmethoden" zitten blokken, Pieter. Waar dat toch van pas kan komen hé :tongue