I use the group-by-group mode.
====== Suggestion 1 =====
[Exit and clear survey] ==> [Logout]
The survey isn't actually cleared, because when I come back later (e.g. using the link in the invitation mail), I see what I entered earlier. The only data that I may loose is the data from the last page (= group).
======= Suggestion 2 ========
"Are you sure you want to clear all your responses?"
==>
"You will be logged out. Have you saved your results?"
The original message may frighten the user, while a [Next] or [Previous] button may have saved the data anyway.
======= Suggestion 3 ========
[Next] ==> [Next (and saved)]
[Previous] ==> [Previous (and saved)]
This clarifies to the user that pressing [Previous] and [Next] is a good way to save intermediate results before going to lunch.
======= Suggestion 4 ========
"One or more mandatory questions have not been answered. You cannot proceed until these have been completed"
==>
"One or more mandatory questions have not been answered. You cannot proceed until these have been completed. (Your intermediate results are saved.)"
This explains to the user that he/she cannot go to the next group, but the data has been saved anyway.
