Privacy
User ID
Also called logged-in id.
User ID is an identifier the site assigns after authentication. It names an account, not a browser.
How it is measured
The site attaches it to events once the person signs in. Check it is a random internal value, not an email or a name.
Count distinct user IDs against active accounts. The difference is the number of accounts that never signed in during the period.
Worked example
A SaaS tool has 1,200 accounts. Analytics shows 1,050 user IDs in the month, so 150 accounts never signed in.
Using the ID, support finds that one customer used the product from three devices, which collapses three client IDs into one person.
How it differs
A user ID names an authenticated person. A client ID names a browser profile. The first excludes anonymous visits; the second excludes the account.
Common errors
Sending raw emails as the ID. Using sequential numbers an outsider can guess. Skipping the notice. Mixing it with client IDs in one column. Keeping it after the account is deleted.
In practice
Use a random internal ID, document where it goes, and delete it with the account.