A backdown from the Government Statistician has revealed one in seven New Zealanders failed to fully complete Census 2018.
Statistics New Zealand chief executive Liz MacPherson has provided the numbers of partial census responses to MPs after repeatedly denying the requests of a parliamentary select committee.
MacPherson has confirmed five per cent of responses were partial, equating to about 240,000 people. When added to the approximate 480,000 who missed the survey, this means 700,000 – or one in seven New Zealanders – did not complete Census 2018.
This gap has no doubt exacerbated the department's attempt to deliver the now exceptionally delayed data set.
Statistics Minister James Shaw, speaking in the House on Tuesday, said he supported MacPherson's refusal to provide the information to the Governance and Administration Select Committee.
The committee had threatened to refer MacPherson for contempt of Parliament on Wednesday after she again refused to provide the partial count number saying it would require "extensive contextual information".
After first saying she would instead provide the answer late April, MacPherson provided a letter detailing the number to MPs on Tuesday.
In the letter, MacPherson said there was "considerable misunderstanding" among select committee MPs and the public about the meaning and relevance of various aspects of the census, including the partial response number.
"These misunderstandings have resulted in unfounded comments regarding the integrity of the official statistics system and Stats NZ."
MacPherson said five per cent of responses to Census 2018 were partial, a jump from two per cent in the 2013 census.
A partial response means Stats NZ received partial information on a person's age, sex, address and relationship to the person completing the survey, and also did not receive a form for every person in a household.
"For example, we may have received a household summary form that indicated that two parents and two teenagers lived in the household, but we did not receive an individual form for one of the children. The information for that child is therefore partial."
New Zealand Initiative chief economist Eric Crampton said the usefulness of census data was "stymied" by the undercount, particularly in measuring the Government's wellbeing agenda.
"If you're not able to match kids to their parents adequately because there's been some missing things in the census then it gets harder to tell the effects of policy on kids."
Independent economist Brian Easton said the gap in census response was now so large the data may be useless for research.
"We have been told that the patching will not all cover all questions which researchers use, while the patching is likely to invalidate many of the issues which researchers want to explore."
National's state services spokesman Nick Smith said hole in the data "will create problems for years in allocating tens of billions of dollars in funding".
"The problems with Census 2018 are so bad that consideration should be given to deferring the electoral boundary changes for 2020 and bringing forward the next Census to 2021.
Speaking in Parliament on Tuesday, Prime Minister Jacinda Ardern declined to comment on the Chief Statistician's stand-off with the select committee, saying the role was independent of Government.
Ardern attributed the issues to funding decisions made during the previous National Government.
"Statistics have told us that they believe they've got the data sets required to produce the information needed by DHBs, needed by Education and needed for the electoral [boundaries]."
National leader Simon Bridges said he was losing faith in Statistics New Zealand and was unwilling to accept funding decisions were to blame.
"It is serious, it's extraordinary to have a situation of Chief Statistician who doesn't want to give information."
An independent review into the handling of Census 2018 is due July 2019.