Researchers can keep participants anonymized yet later recoverable via self-chosen secret numbers

Evidence

Students sometimes sign their names on an “attendance sheet” next to their booklet number. In other studies, I want the participants to be anonymous, yet still be recoverable for future experiments. … You are serving in this experiment anonymously. However, I would like you to put a “secret number” of your own choosing at the top of the bubble sheet. This should be a number you will be able to recognize in a month or so as being yours—but which won’t mean anything to me. Like the last four digits of your phone number, or your best friend’s phone number, or your bike lock combination. So that when I come back to your class to give feedback about the purpose and results of this study, you can come up and get your own personal sheet, with your anonymous feedback on it, and not someone else’s. — Bob Altemeyer, The Authoritarian Specter, 744