Zero-party data

First-party data is what you observed on your own properties: what someone browsed, what they bought, when they came back. Zero-party data is what they told you on purpose — the preference centre they filled in, the quiz they took, the fit or style or budget they declared.

The distinction is real even though the term is invented. Observed behaviour tells you what someone did; declared data tells you what they want, which is often different and is sometimes the opposite. Someone browsing expensive items may be shopping for a gift, or dreaming, or comparing. Ask them and you get an answer no amount of behavioural inference produces.

It also arrives with something valuable attached: the person knowingly gave it, in a context where they could see why. That is a much easier permission to evidence than an inference drawn from tracking.

Two honest caveats.

It is not a legal category. No regulation recognises “zero-party data”. It is personal data, subject to exactly the same rules as anything else you hold, and describing it in a friendlier way does not change what you may do with it.

Declared data decays, and people misreport. Stated preferences go stale as circumstances change, and what someone says in a form is not always what they do at checkout. Treat it as a strong signal, dated, rather than as ground truth — and re-ask rather than assuming the answer from two years ago still holds.