Language Nuance
“As with all employee surveys, your participation is voluntary and your responses are confidential.”
If you read that sentence quickly, you might very well think that the survey is anonymous. However, that is not the case. Instead, the sentence indicates that (a) you can take the survey if you desire and (b) what you say as an individual will not be shared with others who shouldn’t have access.
I’m not too interested in the language about the survey being voluntary. That’s uninteresting to me and it isn’t hard to grok.
On the other hand, I’m very interested in the confidentiality claims. In effect, respondents are giving up information but also their identities. This might include very personal demographic details.
Confidentiality is not equivalent to anonymity. They might be collecting some very juicy stuff.