Indirect Identifier:

A value which cannot be used to identify an individual on its own, but could theoretically be used to identify someone in combination with other values. Indirect identifiers are similar to quasi-identifiers, however some organizations, including Eurostat and Privitar, distinguish between the two on the basis of how likely it is that the value could be used to re-identify an individual.

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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