Here's the link and relevant extract. http://www.cfids-cab.org/cfs-inform/Ptsd/mccue.etal06.pdf Screening for psychological distress using internet administration of the Hospital Anxiety and Depression Scale (HADS) in individuals with chronic fatigue syndrome This would suggest that internet delivery might reduce the effects of social desirability bias and encourage more frank disclosure. On the other hand its difficult to tell what other biases might be introduced. People behave strangely on the 'net'. The may also behave differently when someone has their e-mail address.