Normally you’re happy to get participants from all the ads you post and invitations you send.
In some cases, however, you may want to keep a tree test confidential – for example, if you’re testing the structure of a new site that you want to keep under “double-secret probation” until it launches.
Some tools offer a way to password-protect your study. That is, the participant not only needs the link, but also a password that you supply. If someone stumbles over your study online, or is forwarded the email inadvertently, they still can’t do the study unless they have the password.
If you’re going to use a password, then, it’s good practice to separate the delivery of the link and the password. If you are emailing invitations, for example, send the password in a separate email blast, or ask recipients to reply so you can send them the password.
Next: Writing a good invitation