But the head of the state's curriculum authority, Tom Alegounarias, says it unfair to compare the two credentials and argues the HSC is a more reliable measure of student ability.

While almost 70,000 school leavers received their results before Christmas, a small group of fewer than 500 students have had to wait until Sunday.

The International Baccalaureate (IB) diploma is offered as an alternative to the HSC at about 15 private schools in NSW.

Researchers from the University of Western Sydney compared first-year university students who studied the IB with their non-IB classmates and found they demonstrated greater inquisitiveness, intellectual curiosity, truth-seeking and confidence in reasoning.

The 2014 study, which was commissioned by the IB, also found IB students were more confident in their academic abilities and thought more positively about their academic futures.

"It was pretty clear that the students who did the IB, at least in themselves, feel better prepared for university study," one of the co-authors, associate professor Susanne Gannon, said.

The link existed even when students' background and family education was taken into account.

Mr Alegounarias, president of the NSW Board of Studies, Teaching and Educational Standards, said there was no comparison between the two qualifications.

"The HSC has a level of responsibility to be correct and reliable that no other credential in the state carries and that is what makes it the credential that is chosen by almost all students," he said. "Any marginal activity - homeschooling or the IB - can present itself as an alternative but the HSC is universally regarded as the normal path to recognition."

While some private schools offered the IB "for marketing differentiation", he said, the top students chose the HSC because they wanted the reliability that came with the state credential.

Antony Mayrhofer is the IB diploma co-ordinator at St Paul's Grammar near Penrith, where the year 12 cohort is split evenly between the HSC and the IB.

He has no doubt the IB increases students' capacity to think critically and attributes the relationship to a complusory subject called Theory of Knowledge, a philosophical course assessed through an oral presentation and a 1600-word essay.

"It teaches students to question the nature of knowledge and how knowledge is applied. It puts them in situations where they realise almost all knowledge, if not all knowledge, is biased," he said. "It just makes them question what they're being taught."

Grace Crawford-Smith, an IB graduate from Redlands school in Cremorne, said the course was challenging but rewarding.

"It has a really strong global perspective and a focus on independent learning, which I thought would prepare me for university study," the 18-year-old who hopes to study commerce at the University of NSW, said. "It has definitely made me confident in looking at both sides of all issues you study."

When students receive their results on Sunday morning, their IB scores will range between 24 and 45. Any score above 33 translates to an ATAR above 90.

Last year, 1 in 40 IB students in the state scored the top ATAR of 99.95. By comparison, 1 in every 1150 students HSC students achieved 99.95.

Mr Alegounarias points out the IB is offered at a select few private schools and is taken by "a tiny decimal fraction of the number of students who undertake the HSC.

"The number of students who enter medicine on the basis of HSC alone dwarfs the number of students who do the IB," he said.