Personal data can also be classified according to the extent to which it is personally identifiable. In particular, ISO/IEC19441 distinguishes between five categories including:
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*1.Identified data, which is unambiguously associated with a specific person.
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*2.Pseudonymised data, in which aliases are used in place of personal identifiers; aliases can only be reversed by the party that assigned them.
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*3.Unlinked pseudonymised data, in which all identifiers are erased or substituted by aliases for which the assignment function is erased or irreversible, so that the linkage cannot be re-established by anyone.
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*4.Anonymised data, which is unlinked and altered (e.g. attributes’ values are randomised or generalised) in such a way that there is a reasonable level of confidence that a person cannot be identified.
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*5.Aggregated data, which does not contain individual-level entries and is combined from information about enough different persons that individual-level attributes are not identifiable (ISO/IEC, 2018)