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Definition

Age is the length of time a person has been alive measured in complete, elapsed years. It is measured as the difference between 'date of birth' and 'reference date'. 

Operational issues

The type of question asked to find out someone’s age can affect the reliability of the data. If someone is asked for the date (day, month, and year) of their birth, they are likely to give a more precise answer than if they were just asked for their age. Asking someone for their age (instead of their birth date) can also be confusing for respondents: Does the question refer to their age at their last, next, or nearest birthday? Other causes of error in responses to a direct question on age are:

  • a tendency to report ages ending in certain preferred digits such as zero and five, which can result in misleading age groupings around the five- and 10-year marks
  • exaggeration, especially in older age groups
  • understatement due to a fear of aging or reaching a new decade
  • ignorance of current age
  • carelessness in reporting
  • proxy responses. 

Explanatory notes

Date of birth is often used as an input variable for age rather than collecting age information directly. However, date of birth is not a recognised output variable, but it is used in cohort analysis and exact age calculation.
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