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Operational issues

Operational issues

Derivation of fertility measures

There are three main datasets from which statistical data on fertility are derived. One or more of these datasets may be used to derive a particular fertility measure:

  • birth registrations
  • resident population estimates
  • number of children born alive question.
Birth registrations

Birth statistics are derived from several questions on the birth registration form, called the Notification of Birth for Registration form BDM 27. This form must be completed (generally by the parent(s)) and sent to the Registrar of Births, Deaths and Marriages within two years of the birth. Statistics NZ has the responsibility for processing and publishing births statistics derived from the birth registration form.

Resident population estimates

Population data between census dates is estimated by using the most recent census date as a base. The estimated resident population is updated regularly for population changes due to births, deaths and net migration (arrivals less departures) of residents. Some fertility measures are produced using birth registration data in combination with population estimates. For example, the crude birth rate in a particular year is derived from the number of live births (obtained from the birth registration form) divided by the estimated mean population (obtained from resident population estimates).

Number of children born alive question

A question on the number of children born alive has been asked periodically in several censuses and may be included in other surveys if required. In census, it has been asked of women aged 15 years and over who are resident in New Zealand. It is customary in census to allow respondents the option of objecting to answering the question.

Although slightly different in focus and application, these datasets together provide comprehensive information on fertility. For example, information on the number of children born alive is used by Statistics NZ to supplement fertility studies based on birth registration data. Another use is in formulating fertility assumptions for the population projections produced by Statistics NZ. These are widely used for planning and policy formulation.

Constraints on birth registration data

There are some constraints on the data collected on the vitals' birth registration form. For example, this data provides information on births registered in a particular year. There is usually a time lag between when a birth occurred and when it is registered. The birth registration form must be sent to the Registrar of Births, Deaths and Marriages within two years of the birth, therefore a registration of a birth may potentially be up to two years later than the actual occurrence of the birth.

Births that are not registered within two years can be registered on a separate births register. For example, Section 14 of the Births, Deaths and Marriages Act 1995 refers to births that were not registered in the ordinary way at the time the birth occurred. Such registrations can occur as late as retirement age. These registrations are not included in births statistics derived from the birth registration form.

There may be misunderstanding surrounding the word stillborn. Some respondents may indicate that their child was stillborn assuming that it means that their child is still alive. This may overstate the number of stillbirths recorded in New Zealand.

The birth registration form asks whether there are any other children of the relationship which produced the birth(s) being registered. This parity (number of children born to a mother prior to the current confinement) information is thus restricted to including other children from the same relationship and is not a measure of the lifetime parity of the woman.

Birth registration data cannot be used to analyse childlessness/infertility in the population.

Birth and confinement

Birth statistics derived from the birth registration form may be based on either births or confinements. (A confinement is a pregnancy resulting in one or more children, either liveborn or stillborn.) The number of births will be greater than the number of confinements as a result of multiple births.

Constraints on response to number of children born alive question

The number of children born alive question may be constrained by the respondent's willingness to supply the exact number of live children to whom she has given birth. Some women may feel they are being asked for private information which they would not want disclosed to other family members, for example, ex–nuptial births. Fifteen– or sixteen–year–olds might believe that if they were to answer this question, there could be an investigation into sexual contact below the age of consent. Alternatively, the respondent may have lost a child in infancy and may not want to revive painful memories.

Other respondents may have such strong bonds with stepchildren, adopted or foster children they are caring for that they may want to include them in the question even though they are not their birth mother.

Finally, the number of children born alive question is not currently asked of women aged less than 15 years in census or other surveys, although it is certainly possible that they could have given birth. This only excludes those women who have given birth and at the time of the survey have not turned 15 years of age. Thus, at any point in time, for example, approximately 30 women (based on birth registration figures which show the approximate number of births registered where the mother is aged under 15 years of age) would be affected because they had not yet turned 15 years of age.

Use of fertility terms

Each fertility measure listed in this standard should be referred to by the appropriate term. This ensures that the concept being measured and the population it is based on are clear. For example, the number of children born alive variable should not be referred to as fertility, as this creates confusion with other fertility measures that include the term fertility.

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