Number of children born (information about this variable and its quality)


Number of children born is the number of children ever born to each female aged 15 years and over, who usually resides in New Zealand. Stepchildren, adopted children, foster children, and wards of the State are not included.

The word 'alive' has been removed from this question for 2018. Foetal deaths and stillborn children may now be included by respondents. However, the intent of the question is still the same, and respondents were not specifically asked to include foetal deaths and stillbirths.



Variable Details

Other Variable Information

Priority level

Priority level 3

We assign a priority level to all census variables: Priority 1, 2, or 3 (with 1 being highest and 3 being the lowest priority).

Number of children born is a priority 3 variable. Priority 3 variables do not fit in directly with the main purpose of a census but are still important to certain groups. These variables are given third priority in in terms of quality, time, and resources across all phases of a census.

The priority level for number of children born remains the same as 2013.

Quality Management Strategy and the Information by variable for number of children born (2013) have more information on the priority rating.

Overall quality rating for 2018 Census

Moderate quality

Data quality processes section below has more detail on the rating for this variable.

Subject population

Female census usually resident population aged 15 years and over.

‘Subject population’ means the people, families, households, or dwellings to whom the variable applies.

How this data is classified

Number of children bornV1.0.0

Number of children born is a flat classification with the following categories:

000 No children

001 One child

002 Two children

003 Three children

004 Four children

005 Five children

006 Six children

007 Seven children

008 Eight children

009 Nine children

010 Ten or more children

555 Object to answering

999 Not elsewhere included

‘Not elsewhere included’ contains the residual categories of ‘response unidentifiable, ‘response outside scope’, and ‘not stated’.

The classification of number of children born in the 2018 Census is consistent with both the 2013 Census and the 2006 Census.

The Standards and Classifications page provides background information on classifications and standards.

Question format

Number of children born is collected on the Individual form (question 27 on the paper form)

Stats NZ Store House has samples for both the individual and dwelling paper forms.

There was a change to the question wording from the 2013 Census:

  • in 2013, after the question ‘how many babies have you given birth to’ the box where respondents provided their answer was labelled ‘number born alive’. For the 2018 Census, this was changed to ‘number born’.

There were differences in the way a person could respond between the online and paper versions of the form.

On the online Individual form:

  • number of children born had to be answered with a single response for the respondent to submit the form
  • built-in routing functionality directed females aged 15 years and over to the question. Males and females aged less than 15 years were routed past the question.

On the paper Individual form:

  • those outside the subject population could respond to the question (such as males, those under 15, and non-residents). No change was made to these as they will be filtered out by using the correct subject population.
  • multi-responses were possible (for example, ticking both none and a number of children). Multi-responses were edited to retain one valid response.

Data from the online forms may therefore be of higher overall quality than data from paper forms. Data quality processes section has more information on the effect of survey mode on data quality for this variable.

How this data is used

Outside Stats NZ

  • To aid research and policy by providing information on population structure and change.
  • As an indicator of changes in rates of childbearing and childlessness over time.

Within Stats NZ

  • To improve the quality of population projections.
  • To analyse family sizes, childlessness, and fertility rates among different populations and ethnic groups.

2018 data sources

We used alternative data sources for missing census responses and responses that could not be classified or did not provide the type of information asked for. Where possible, we used responses from the 2013 Census, administrative data from the Integrated Data Infrastructure (IDI), or imputation.

The table below shows the breakdown of the various data sources used for this variable.

2018 Number of children born – female census usually resident population aged 15 years and over
Source Percent
Response from 2018 Census 85.6 percent
2013 Census data (females aged 45+ in 2013 only) 2.8 percent
Administrative data (females aged 10–45 in 2013 only) 4.1 percent
Statistical imputation 0.0 percent
No information 7.5 percent
Total 100 percent
Due to rounding, individual figures may not always sum to the stated total(s)  

2013 Census data was only used for missing or residual responses for females who were aged over 45 years old in 2013, as they have a lower chance of having given birth since the 2013 Census.

The ‘no information’ percentage is where we were not able to source number of children born data for a person in the subject population.

Administrative data sources

Data from the following administrative source was used:

  • Births register, Department of Internal Affairs.

Comparing 2013 Census and admin data for number of children born has more information.

Please note that when examining number of children born data for specific population groups within the subject population, the percentage that is from 2013 Census data and administrative data may differ from that for the overall subject population.

Missing and residual responses

‘No information’ in the data sources table is the percentage of the subject population coded to ‘not stated’. In previous censuses, non-response was the percentage of the subject population coded to ‘not stated’.

Percentage of ‘not stated’ for the female census usually resident population aged 15 years and over:

  • 2018: 7.5 percent
  • 2013: 7.5 percent
  • 2006: 6.7 percent.

Responses that could not be classified or did not provide the type of information asked for such as response unidentifiable and response outside scope remain in the data where we have been unable to find information from another source. In the data sources table, these residuals are included in the ‘Response from 2018 Census’ percentage.

For output purposes, these residual category responses are grouped with ‘not stated’ and are classified as ‘not elsewhere included’. In 2018, there were only a small number of responses outside scope, so the percent of not elsewhere included (to 1 decimal place) matches the percent of ‘not stated’.

Percentage of ‘not elsewhere included’ for the female census usually resident population aged 15 years and over:

  • 2018: 7.5 percent
  • 2013: 7.6 percent
  • 2006: 7.0 percent.

2013 Census data user guide provides more information about non-response in the 2013 Census.

Data quality processes

Overall quality rating: Moderate quality

Data was evaluated to assess whether it meets quality standards and is suitable for use.

Three quality metrics contributed to the overall quality rating:

  • data sources and coverage
  • consistency and coherence
  • data quality.

The lowest rated metric determines the overall quality rating.

Data quality assurance for 2018 Census provides more information on the quality rating scale.

Data sources and coverage: Moderate quality

We have assessed the quality of all the data sources that contribute to the output for the variable. To calculate a data sources and coverage quality score for a variable, each data source is rated and multiplied by the proportion it contributes to the total output.

The rating for a valid census response is defined as 1.00. Ratings for other sources are the best estimates available of their quality relative to a census response. Each source that contributes to the output for that variable is then multiplied by the proportion it contributes to the total output. The total score then determines the metric rating according to the following range:

  • 98–100 = very high
  • 95–<98 = high
  • 90–<95 = moderate
  • 75–<90 = poor
  • <75 = very poor.

Data from the 2013 Census was highly comparable to census forms, while admin data was highly comparable to census forms with the following caveats:

  • DIA Births Register captures New Zealand births only and not children born to migrants before they emigrated to New Zealand
  • the Births Register does not capture childlessness.

The proportion of data from received forms, 2013 Census data, and admin data, along with the percentage of the dataset remaining as ‘no information’ contributed to the score of 0.92, determining the moderate quality rating.

Quality rating calculation table for the sources of number of children born data –
2018 female census usually resident population aged 15 years and over
Source Rating Percent of total Score contribution
2018 Census form 1.00 85.63 0.86
2013 Census 0.96 2.81 0.03
Admin data 0.97 4.10 0.04
No Information 0.00 7.46 0.00
Total 100.00 0.92
Due to rounding, individual figures may not always sum to the stated total(s) or score contributions.      

Consistency and coherence: High quality

Number of children born data is consistent with expectations across nearly all consistency checks, with some minor variation from expectations or benchmarks that makes sense due to real-world change, incorporation of other sources of data, or a change in how the variable has been collected.

The number of ‘object to answer’ responses has decreased from previous censuses. This may be due to replacing ‘born alive’ with ‘born’.

Data quality: High quality

The data quality checks for number of children born included edits for consistency within the dataset and cross-variable checks to the regional council level.

Number of children born data has only minor data quality issues. The quality of coding and responses within classification categories is high. Any impact of other data sources used is minor. Any issues with the variable appear in a low number of cases (typically in the low hundreds).

Recommendations for use and further information

While 2013 Census and administrative data contribute to the production of the 2018 Census data, the overall quality of the data is moderate and comparable with 2006 and 2013 data.

However, when using this data you should be aware that:

  • due to the use of administrative enumeration to replace missing responses in the 2018 Census, the ethnicity counts in the 2018 Census are generally higher than in the 2013 Census, including for the Māori ethnic group. As a result of this change in methodology for the 2018 Census, number of children born time series data for the Māori ethnic group should be interpreted with care.
  • data has been assessed to be consistent at the regional council level of geography. Some variation is possible at geographies below this level.
  • data has been checked against level 1 ethnicity and found to be consistent with expectations. Some variation is possible at ethnicities below level 1 of the classification.

Comparisons with other data sources

Census is the only comprehensive source of information about this variable.

Contact our Information Centre for further information about using this variable.

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