In multiple studies it was found that people are more likely to lie when writing an email, then with pen and paper.
People are also more negative via email versus pen and paper.
However, people lie the most when on the phone.
In a self-reporting diary study, participants said they were most likely to lie on the phone, least likely to lie over email, and face to face and instant messaging fell equally in the middle of the former two.
Slide 2
Conclusion
If you are sending out a survey via email remember that people will tend to be more negative.
Surveys through phone calls are not always accurate, as people are more likely to lie over the phone.
For the best results on a survey, they should be done one-on-one and in person.