From: A randomized trial of mailed questionnaires versus telephone interviews: Response patterns in a survey
Item
Don't-knows (%)
Telephone
Mail
I seem to get sick a little easier than other people
3.33
10.22
I am as healthy as anybody I know
2.69
13.90
I expect my health to get worse
4.36
23.76
My health is excellent
2.05
10.39