Number of motor vehicles (information about this variable and its quality)

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 motor vehicles 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 terms of quality, time, and resources across all phases of a census.

The census priority level for number of motor vehicles remains the same as 2013.

Quality Management Strategy and the Information by variable for number of motor vehicles (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.

The External Data Quality Panel has provided an independent assessment of the quality of this variable and has rated it as moderate quality. 2018 Census External Data Quality Panel: Assessment of Variables has more information.

Subject population

Households in occupied private dwellings.

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

How this data is classified

Census number of motor vehicles classification V2.0.0

Number of motor vehicles is a flat classification with the following categories:

0 No motor vehicles

1 One motor vehicle

2 Two motor vehicles

3 Three motor vehicles

4 Four motor vehicles

5 Five or more motor vehicles

9 Not elsewhere included

‘Not elsewhere included’ contains the residuals ‘response unidentifiable’ and ‘not stated’.

Although there have been no conceptual changes to this variable, there have been minor changes to the classification of this variable since the 2013 Census:

• the category of ‘three or more motor vehicles’ has changed to ‘three motor vehicles’
• two additional categories have been added: four motor vehicles, five or more vehicles.

The information by variable for number of motor vehicles (2013) has more information.

Number of motor vehicles can be grouped for output as follows:

• using a top category of three or more, or four or more
• adding a subtotal for households with access to at least one motor vehicle, or using summary categories of ‘no access to a motor vehicle’ and ‘access to a motor vehicle’.

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

Question format

Number of motor vehicles is collected on the dwelling form (question 13 on the paper form).

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

There were no differences between the wording or question format in the online and paper version of this question.

There were differences in the way a person could respond:

On the online dwelling form:

• only one response could be selected. If ‘none’ was selected, any numerical responses given previously would disappear
• the numeric response box accepted values of up to 99.

On the paper dwelling form:

• non-response to this question was possible
• responses outside the valid range and multiple responses were possible. These were resolved using edits.

How this data is used

Outside Stats NZ

• By local government and transport planners to plan transport services.
• To assist in the location of public transport services, commercial, and welfare facilities.
• To study energy conservation.
• In developing the New Zealand Deprivation Index.

Within Stats NZ

• District Health Board and regional summary tables.
• Time series tables and cross-tabulated with geographic variables, household composition, number of usual residents in a household, and household income.

2018 data sources

The table below shows that there were no alternative data sources or imputation used to replace missing responses or responses that could not be classified for number of motor vehicles.

The ‘no information’ percentage is where we did not have a response to the number of motor vehicles question and there were no alternative data sources for a household in the 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 households in occupied private dwellings:

• 2018: 7.8 percent
• 2013: 5.0 percent
• 2006: 4.1 percent.

Responses that could not be classified or did not provide the type of information asked for (response unidentifiable) remain in the data, where we have been unable to find information from another source. In the data sources table, this residual is included in the ‘Response from 2018 Census’ percentage.

Percentage of ‘not elsewhere included’ for households in occupied private dwellings:

For output purposes, as with the 2013 and 2006 Censuses, these residual category responses are grouped with ‘not stated’ and are classified as ‘not elsewhere included’.

Due to the small number of residuals other than ‘not stated’, the ‘not elsewhere included’ percentage for 2013 and 2006 was the same as ‘not stated’.

• 2018: 8.3 percent
• 2013: 5.0 percent
• 2006: 4.1 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. As no alternative data sources or imputation were used to replace missing responses, the final data sources and coverage quality rating for number of motor vehicles reflects the lower than expected response to the census overall and the percentage that did not answer the number of motor vehicles question.

The rating for a valid census response is defined as 1.00. This is then multiplied by the overall proportion of responses from 2018 Census forms. 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.

The proportion of data from received forms therefore contributed to the score of 0.92, determining the moderate quality rating.

Consistency and coherence: High quality

Number of motor vehicles in the 2018 Census is highly comparable with the 2013 and 2006 Census data. Variable data is also consistent with expectations across nearly all consistency checks, with only minor variation from expectations.

There has been a significant increase in the ‘response unidentifiable’ category for this variable. This is due primarily to scanning misrecognition and facetious answers. The ‘not stated’ category has also increased since previous censuses and was not able to be replaced using alternative data sources.

The data between 2013 and 2018 shows a greater rate of change across most motor vehicle count categories than occurred between 2006 and 2013. Although this change appears inconsistent with previous data, it is consistent with data from other sources Environmental Health Indicators New Zealand – see Figure 1. We observed a decrease in ‘none’ and ‘one’ motor vehicle responses and an increase in three of more motor vehicles. Potentially the increase in access to three or more vehicles may be the result of real-world change in people’s living arrangements. Younger people could be staying at home longer, more students/young people flatting together in a single household, or multiple families living in a single household.

Data quality: High quality

The data quality checks for number of motor vehicles included edits for consistency within the dataset and cross-tabulations to the regional council geographic level.

Number of motor vehicles has only minor data quality issues. The quality of coding and responses within the classification categories is high. Any issues with this variable appear to be in a low number of cases (typically in the low hundreds).

Recommendations for use and further information

The overall quality of the number of motor vehicles data is moderate and comparable with 2006 and 2013 Census data.

The data is of sufficient quality to extract exact motor vehicle counts, which was not previously possible.

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

• time series trends across regional councils are mostly consistent with those observed at a national level. But caution should be taken when looking at exact counts of motor vehicles in some smaller areas, as response rates may be lower in some areas compared with the national average.
• for 2018, this variable has a higher non-response rate than previous censuses, so care should be made when interpreting trends.
• there has been a minor classification change to the variable, as two additional categories were added to the classification (four motor vehicles and five or more motor vehicles). For time series analysis, the 2018 Census data has been concorded to the previous classification. Where there was a change to the classification, there were no noticeable impacts to coding compared with the 2013 Census data.

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