Anonymous and confidential do not mean the same thing. “Anonymous” means you do not know who provided the responses. “Confidential” means you know or can find out who provided the responses, but you are committed to keeping the information to yourself.
You must ensure that you protect the confidentiality of any individual’s data or comment. It is easy to make your surveys anonymous, but if you want to track people over time, you’ll likely need to attach ID numbers to each person, keeping a list of the names and numbers in a locked file.
It is important to inform people that you are measuring your program’s outcomes and may use data they provide in some way. You must let them know that their participation is voluntary and explain how you will maintain the confidentiality of their data.