A data value that doesn’t identify an individual on its own but can become identifying in combination with other quasi identifiers. Quasi identifiers aren’t direct identifiers. Instead, they are identifiers such as an area code or zip code or your date of birth. There will be many people who share a zip code, and many people who share a date of birth but not very many who share both.
There are some organizations who use the term interchangeably with indirect identifiers. But, there are others (such as Eurostat and Privitar), that differentiate between the two terms. The difference between an indirect identifier and a quasi identifier is that quasi identifiers are values that are reasonably likely to be available in other sources and so could be used to link two datasets.
For example, date of birth and postcode would be considered quasi identifiers, as they are quite likely to be known by others and so could be used in a Linkage Attack. Whereas a value not widely known, for instance an internally generated value like a payment plan category, might be considered an indirect identifier. All values that are not direct identifiers are indirect identifiers, but not all indirect identifiers are quasi identifiers.
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