Preventing duplicate survey responses

The only 100% secure method of preventing duplicates is to issue individual passwords or tie completions to some other unique value such as a receipt #, user account, etc. Just remember simple passwords are best.

For customer surveys, event or training feedback, and similar topics, this sort of identification is generally not an issue—you can simply e-mail individual passwords to your list.

For market research, if you use a panel vendor your organization can remain anonymous while still providing compensation.

For employee satisfaction and other sensitive surveys, you may need to forgo the restriction or use a third party consultant or host who will assure anonymity. You can also issue passwords in a way that isn't tied to an e-mail invitation, such as picking a slip of paper or having people checked off as they arrive at a set of kiosks to complete.

When a pre-issued code isn't an option and you do think people will stuff the ballot box, here are some strategies you can use:

• With per-respondent compensation, make the payout only after data has been validated. The more contact information you collect, such as a physical mailing address, the harder it is for respondents to pretend to be someone new.
• Cookies, where you write to the respondent's computer that they've answered, can work as a casual barrier. It's only casual because cookies can be blocked or cleared, and are tied to a specific browser.
• IP addresses, the server ID through which the respondent is surfing, are sometimes limited to one submission. The catch is users coming out of company networks or some ISPs will often be sharing the same IP address, so you may be blocking new respondents along with repeat submissions.