Personal data
PII — personally identifiable information
Personal data is any information relating to an identified or identifiable natural person, directly or indirectly, including by cross-referencing with other information.
The test is not the name, it is identifiability
A name is personal data, but so are an IP address, an advertising identifier, a customer number or a browser fingerprint: the test is not "can we read a name?" but "can we, by reasonable means, get back to a person?". Cross-referencing counts: two harmless pieces of information can become identifying together.
Sensitive data is a category of its own
Article 9 of the GDPR sets apart special categories — origin, political opinions, religious beliefs, health, sexual orientation, biometric data — whose processing is prohibited in principle, save for exceptions. An audience measurement tool should never collect any of it, neither directly nor inadvertently through a URL or a form field.
In audience measurement
An IP address is personal data: that is why solutions built for consent exemption use it transiently, to derive a country or a region, and never keep it in identifying form. The most common blind spot is not the tracker itself but URLs: an email address or a customer id slipped into a URL parameter enters the tool along with everything else.