I think all of the following (and more) are possible risks:
- People are tired/bored and so answer less effortfully/more quickly
- People are annoyed and so answer in a qualitatively different way
- People are tired/bored/annoyed and so skip more questions
- People are tired/bored/annoyed and dropout entirely
Note that people skipping questions/dropping out is not merely a matter of quantity (reduced numbers of responses), because the dropout/skipping is likely to be differential. The effect of the questions will be to lead to precisely those respondents who are more likely to be bored/tired/annoyed by those questions and to skip questions/dropout if bored/tired/annoyed to be less likely to give responses.
Regrettably, I think that specifying extremely clearly that the questions are completely optional influences some respondents (it also likely makes many simply less likely to answer these questions), but doesn’t ameliorate the harm for others. You may be surprised how many people will provide multiple exceptionally long open comments and then complain that the survey took them longer than the projected average. That aside, depending on the context, I think it’s sometimes legitimate for people to be annoyed by the presence of lots of open comment questions even if they are explicitly stated to be optional because, in context, it may seem like they need to answer them anyway.
I think all of the following (and more) are possible risks:
- People are tired/bored and so answer less effortfully/more quickly
- People are annoyed and so answer in a qualitatively different way
- People are tired/bored/annoyed and so skip more questions
- People are tired/bored/annoyed and dropout entirely
Note that people skipping questions/dropping out is not merely a matter of quantity (reduced numbers of responses), because the dropout/skipping is likely to be differential. The effect of the questions will be to lead to precisely those respondents who are more likely to be bored/tired/annoyed by those questions and to skip questions/dropout if bored/tired/annoyed to be less likely to give responses.
Regrettably, I think that specifying extremely clearly that the questions are completely optional influences some respondents (it also likely makes many simply less likely to answer these questions), but doesn’t ameliorate the harm for others. You may be surprised how many people will provide multiple exceptionally long open comments and then complain that the survey took them longer than the projected average. That aside, depending on the context, I think it’s sometimes legitimate for people to be annoyed by the presence of lots of open comment questions even if they are explicitly stated to be optional because, in context, it may seem like they need to answer them anyway.