Children born in the later half of each year have a better chance of doing well at high school years later, a new study suggests.

A prize-winning study by University of Canterbury economists Asaad Ali and Andrea Menclova has found that a child's chances of achieving University Entrance (UE) increase by about 5 per cent for every extra month spent in Years 0 and 1.

Children born in June have the highest chances of achieving UE because many schools put them into Year 0 for the rest of the year in which they turn 5, and then give them a whole Year 1 starting the next February.

Children born in May have the lowest chances of later achievement because they often go straight into Year 2 the next February, giving them about half a year less time in primary school than children born a month later.

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The unique study was made possible by the New Zealand system in which most children start school as soon as they turn 5.

In most other developed countries, children all start school at the start of the next school year after they turn the required age, so they all receive the same amount of primary schooling.

However similar findings to the NZ study have been found for the Netherlands, where most children start school as soon as they turn 4.

The paper uses Statistics NZ's integrated data infrastructure (IDI), which allows researchers to trace individuals anonymously through multiple data sets. It won a Statistics NZ prize for "best use of official statistics" at this year's Association of Economists conference.

Asaad Ali (left) and Associate Professor Andrea Menclova (centre) received an award from Statistics NZ chief methodologist Vince Galvin. Photo / Supplied
Asaad Ali (left) and Associate Professor Andrea Menclova (centre) received an award from Statistics NZ chief methodologist Vince Galvin. Photo / Supplied

Ali, a doctoral student from Pakistan, said the topic was chosen by Menclova, his Czech-born supervisor and an associate professor of economics.

"This is the first time I have done research in education economics," he said. "My thesis for my Master's was in banking, in Pakistan."

The pair did not have data on when children actually started school, so their calculations are based on the time that they potentially started, and hence the amount of time they potentially spent in primary school.

They found that, for NZ-born children who left secondary school between 2009 and 2016, every extra month of potential time in primary school lifted their chances of achieving the National Certificate of Educational Achievement (NCEA) Level 1 by 2 per cent - after allowing for decile and other factors.

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Each extra month also lifted their chances of achieving NCEA Level 2 by 4 per cent, NCEA Level 3 by 6 per cent and UE by 5 per cent.

Later birthdays made slightly more difference for boys than for girls, but there was no clear pattern by ethnicity or decile, with the strongest effects tending to be in mid-decile schools.

The authors tested their findings against migrant and refugee children, finding that their birthdays made no difference because most started school in countries where children all started together at the start of each school year.

But international fee-paying students, who were also born overseas, showed similar birthday effects as NZ-born students - suggesting that the birthdays might actually be purely random variables.

Auckland Primary Principals' Association president Heath McNeil said schools were free to set their own policies about when children were classified as Year 0 or Year 1.

He said the cutoff for Ministry of Education funding was July 1. Five-year-olds starting before July 1 are classed as Year 1 and those starting after that date are classed as Year 0 in the rest of that year for funding purposes.

"So some schools use that cutoff, but many others use their own dates between May and June," he said. "Others use the end of Term 1 as their cutoff."

'Teachers make the difference'

Tianna Steele (left) has a statistically better chance of doing well in high school because her birthday is in September, compared with her sister Sienna (right), born in March. Photo / Michael Craig
Tianna Steele (left) has a statistically better chance of doing well in high school because her birthday is in September, compared with her sister Sienna (right), born in March. Photo / Michael Craig

Statistically speaking, Tianna Steele has a better chance of getting University Entrance than her sister Sienna because Tianna's birthday is later in the year.

Tianna started school as soon as she turned 5 in September 2017, so she had three months in Year 0 before doing Year 1 last year and is now in Year 2.

Sienna will start school when she turns 5 next March, so she will probably get slightly less than a full year in Year 1 next year before she goes on to Year 2 at the start of 2021.

But their grandmother Christine Allan, who took them on a school-holiday outing to Auckland's Sky City yesterday, is sceptical about whether Tianna's extra four months or so of primary schooling will really make much difference to her later school achievement.

"Well yes, it's a pretty good theory - in theory," she said.

"I'm not too sure about that, to be honest. I think it's more either the teacher or smaller classes than how much time they have in school."

Tianna said there were 18 or 20 other students in her class when she started at Rutherford Primary School in Te Atatū.

"We became friends," she said.

By contrast, her grandmother can remember classes of 40 at her primary school in Waitara in the 1960s.

"Back then when I was at school, it wasn't that wonderful," she said.

"It depended on the teachers, to be honest. If you have a good teacher it makes all the difference.

"I'm speaking from experience. I'm from a big family, there were 11 of us and none of us did very well at school.

"They [her grandchildren's generation] have got fantastic teachers. They really do make a difference, they are more caring. I think it was just a job before."