Age

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 'data collection date'.

Operational issues

The reliability of age data may be affected by the type of question asked. Asking for day, month and year of birth has been shown to yield data that are more precise because the question is so specific it discourages answers in approximate terms.

Responses to the question on age at last birthday are likely to be less accurate than responses to the question on date of birth. Independent of their method of reckoning age, respondents may be unsure whether the age wanted is that at 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, hence age clustering at five and ten year intervals;
• 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.

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. For this reason, both variables are discussed within the one standard.