Quasi-identifiers (also called indirect identifiers) are information that does not by itself identify a specific person, which is the case with PII (personal identifiable information). However, problems arise from deriving PII from a combination of multiple quasi-identifiers together provide a clue to identify a person. As an example, it has been shown that 63% of the US population is uniquely identifiable only by a combination of gender, date of birth and zip code.