Age

Classification criteria

The criterion used to place a person into the classification is the elapsed time between birth and a data collection, calculated in whole years.

Classification

Age classification

Age is a flat classification with single–year categories from 0 years to 119 years inclusive, plus a category for 120 years and over.

The full classification is available in Download of classification.

No provision is made for residual categories as, in line with international practice, it is Statistics New Zealand policy to impute missing values for age data. The reasoning for this is due to the practicalities of data presentation. Age is a fundamental variable used in a variety of published cross–tabulated outputs. Due to space limitations in hardcopy publications, and to simplify the presentation of data for users (eliminating an over abundance of residual categories in tabulated outputs), residual categories are removed by imputing any missing age data.

Date of birth classifications

Classifications for date of birth are required because age is derived from the information collected by such a question. Date of birth is derived from three flat classifications; date of birth day, date of birth month and date of birth year. Day has single categories 01 to 31 (representing the days of the month). Month has single categories 01 to 12 (representing the months of the year). Year has single categories (representing 120 years back from the current year). For example the 2007 code structure for the year classification ranges from 1887 to 2007. All three classifications also have residual categories.

The residual categories are defined in Glossary and references.

The full classification is available on the main page.

Coding process

Age should be derived from date of birth, see the standard derivation routine below.

If values are missing, age is to be imputed.

Derivation of age

1. Where day, month and year of birth are all specified, the respondent’s true age is calculated as a whole year with respect to the reference day.
2. Where only month and year of birth are specified, and the month specified is not the reference month, age can again be calculated as a whole year with respect to the reference day.
3. Where only month and year of birth are specified, and the month equals the reference month, compute X=(reference day)/(number of days in reference month). Generate a random number between 0 and 1. If the random number is less than X, day of birth is assumed to fall before reference day. If the random number is greater than or equal to X, the day of birth is assumed to fall on or after reference day.
4. Where only year of birth is specified, a process similar to step 3 is followed, with X being computed as (reference day, specified as number of days since 1 January in reference year)/(total number of days in reference year).
5. Where only day and year of birth are specified, a random number between 1 and 12 inclusive is generated for the month. Age can then be calculated as a whole year, with respect to the reference date.
6. Where only day and month of birth are specified or none of the three fields are given or the values specified are later than the reference date, then age is imputed.