Anonymous survey definition:
An anonymous survey is a survey conducted by an objective researcher and excludes the collection of respondent's personal information so that whoever filled out the survey, his or her choices can't be identified later on. It is helpful to define the terms “anonymous” and “confidential.” Anonymous means not identified by name. Confidential means private, secret, not universally available or known only to a select few.
In an anonymous survey, any information identifying respondents is encrypted. For web surveys, this identifying information is the IP address of the computer running the web browser. For email surveys, this is the email address of the person who sends the survey. The encrypted identifying information is stored in the HOST field of your database.
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How do you make sure a respondent only takes the survey once?
If the system is fully anonymous (to the extent where there's nothing being tracked about the respondent), then you are essentially allowing one respondent to take the survey as many times as they want, thus ending up with possible biased data.
If you want to make sure that one person can take the survey only once, then you need some kind of tracking or identification system to see who already took the survey and who hasn't.
Luckily, you can prevent people from taking the survey twice, without knowing who they are. Remember those encrypted values? Duplicate addresses will show the same encrypted value allowing you to filter out duplicates easily and ensuring unbiased data.
Having said that, the aim of an anonymous survey isn't to have not identifiable people but to have not identifiable answers. Where we can't tell what each respondent reply, but we do know who has and who hasn't completed the survey.
Anonymous survey example:
Let's consider polls. They can be anonymous to the extent you can know who is voting, but you would not know who they voted for. So, in such cases, you can track the IP or e-mail address, but you cannot track the voting results (their choices).