> You cannot ignore the group that didn't answer the questionnaire, as they will most likely expose some of the behavior that you are researching (i.e. about life etc), and might have a huge impact on your results.

This is completely paradoxical.

You are saying that using the data from the 90 would be jumping to conclusion because you would probably ignore data that would not match those 90.

But making this claim IS jumping to conclusions, because you are making an assumption (the 30 have something in common explaining why they didn't fill the form).