A microdata file V with s respondents and t attributes is an s × t matrix where Vij is the value of attribute j for respondent i. Attributes can be numerical (e.g., age, salary) or categorical (e.g., gender, job).
Identifiers. These are attributes that unambiguously identify the respondent. Examples are the passport number, social security number, name-surname, etc.
Quasi-identifiers or key attributes. These are attributes which identify the respondent with some degree of ambiguity. (Nonetheless, a combination of quasi-identifiers may provide unambiguous identification.) Examples are address, gender, age, telephone number, etc.
Confidential outcome attributes. These are attributes which contain sensitive information on the respondent. Examples are salary, religion, political affiliation, health condition, etc.