If all hospitals performed to best-case level, hundreds of lives would be saved each year.
Hundreds fewer patients would die each year if the death rates at all public hospitals matched the best performers.
But every year several district health boards have a death rate in patients admitted to their hospitals that may be higher than the national rate, and this is regular feature for some, including the Waikato DHB.
So-called "in-hospital" deaths within 30 days of admission number nearly 8000 patients each year, around 1.5 per cent of all people admitted to hospital, including day patients.
Most die from the heart attack, pneumonia or other disorder which landed them in hospital. But in a minority of cases - one in 20 deaths, according to overseas research - it is thought the person's death could have been avoided, by things such as more-careful medicines management or the use of standardised methods to avert falls or infections.
Ministry of Health data obtained by the Herald under the Official Information Act shows that although the number of deaths of inpatients in public hospitals has increased by 20 in the five years to June last year, the mortality rate, which takes into account the growing volume of patients treated, has declined to 1.48 per cent.
It was 1.68 per cent in 2006/7.
At that time, Professor Alan Merry and Dr Mary Seddon wrote in the New Zealand Medical Journal of the "epidemic" of healthcare-induced harm. "... our hospitals are not acceptably safe at present ..."
Six years later, Professor Merry, now chairman of the Health Quality and Safety Commission, said DHBs had since shown a strong and increased commitment to quality and safety improvement.
"I am confident safer services are now being offered to our patients.
"However ... there is always potential to do better. We still see preventable errors happening in our hospitals."
New Zealand health authorities, unlike some overseas, have not generally promoted comparisons of public hospitals' death rates, although brief details of district health boards' "serious and sentinel events", including potentially avoidable patient deaths, have been published since 2008.
The exception is death rates of newborns and stillbirths, on which a Government committee publishes annual data comparing health boards.
DHBs refused to give the Herald hospital mortality reports produced for them by the Australia-based Health Roundtable.
In Canada and Britain, data are readily available comparing groups of hospitals on mortality rates, and in Australia, health ministers have endorsed using this kind of information as a key indicator of quality and safety.
In New Zealand it was the landmark study by Professor Peter Davis and colleagues that jolted health administrators into understanding the need to improve safety and quality.
Broadly in line with overseas findings, the 2002 study found that 1.9 per cent of New Zealand hospital admissions were associated with an adverse event that resulted in permanent disability or death. Some of the events occurred prior to admission, but most happened in hospital. More than a third were preventable to a significant degree.
In the ministry-produced standardised mortality estimates, each DHB's in-hospital death rate is weighted either up or down in line with the national population, to allow fairer comparisons than with the raw data.
"When standardising," said National Health Board national director Chai Chuah, "the mortality rates are essentially weighted by the person's age, gender, rurality, ethnicity and deprivation. It also takes into account whether the event was an acute admission or not, the complexity of the [person's illness or injury] and the number of operations."
Waikato DHB had the highest rate for three of the five years and was never below third-highest.
The Auckland region's three health boards have consistently had mortality rates in the lower half of DHBs and the Auckland DHB and Waitemata rates have tended to be particularly low.
The standardised rates are estimates and there is considerable overlap once the statistical uncertainties are taken into account, but "statistically significant" differences remain.
For instance, the Auckland and Southern DHBs' rates were unequivocally lower than Canterbury's, Bay of Plenty's and Waikato's in 2010/11. And Waikato was above the national average in three years.
The upper North Island's smaller-population DHBs varied widely during the five years: Northland from ninth-highest, to lowest; Bay of Plenty from second-highest to eighth; and Lakes DHB from fourth-highest, to 10th.
Canterbury and Bay of Plenty may have been above the national average in 2010/11. If they and Waikato had matched the average, those districts collectively would have had around 300 fewer deaths, and even fewer if they had matched the best performer, the Southern DHB.
Ministry chief medical officer Dr Don Mackie wouldn't directly answer whether there was concern over the outlier rates like Waikato's, but said it was a high-level indicator that warranted further investigation.
"What's really reassuring is that Waikato is doing a lot of work to understand this themselves - the causes, whether they lie in the service mix, the population mix, the fact that Waikato is made up of a number of hospitals may feed into that, something about the way clinical services are run.
"Waikato are currently screening 99.8 per cent of deaths; they are finding about 2 per cent of them are worthy of further investigation.
"What we are looking for is whether there is any way to improve that death rate by looking at avoidable death. Some of the answer may well lie in patterns of admission, disease prevalence in the community.
"Waikato are the first DHB to put out a Quality Report, which is reassuring and it shows they are paying attention to these issues and as a DHB have a strong quality focus."
He said not all jurisdictions internationally recorded in-hospital mortality in the same way, making comparisons difficult, but Australian states tended to have a similar method and rate to New Zealand's.
The OECD compares members' in-hospital mortality rates for several illnesses, including acute heart attack, for which New Zealand was markedly better than the average in 2009, and stroke, for which it might have been slightly worse than average.
The use of standardised mortality rates to compare whole hospitals and assess healthcare quality is controversial. Some say the statistics are not sensitive enough to be of much use because quality of care accounts for only a small fraction of the variance.
They say a hospital's rate may appear better than it is because of factors such as good access to non-hospital palliative care for the dying, meaning fewer deaths occur in hospital. And they warn that comparisons based on the rates may lead to subtle data manipulation and inappropriately aggressive health care.
"The link between [the rates] and other measures of quality has been weak or non-existent," said Dr Seddon, head of quality improvement at Counties Manukau DHB.
The ministry, however, is happy to claim a link to quality.
Its 2011 annual report says hospital mortality rates are analysed to identify outliers, "in order to focus on quality of care. [New Zealand's] rates have been declining since 2000/01, suggesting improved quality of care on this outcome measure."
In work now overseen by the Health Quality and Safety Commission, numerous DHB-based groups and several national committees have improved the analysis of deaths, looking for lessons, in areas such as child and youth mortality, perinatal and maternal mortality, and surgical mortality. The commission also supports DHBs' efforts to:
Spread the use of a standardised safe-surgery checklist.
Reduce falls in hospitals.
Improve medication safety.
Increase hand hygiene.
Reduce blood infections from catheters in veins
Produce Waikato-style quality accounts.
In the commission's latest serious and sentinel events report, Professor Merry said the aim of the reporting system was to enable health workers to learn from the events and make the health system progressively safer.
"Obviously this is not easy, otherwise these events would already have been eliminated."
Five years of hospital death rates have been made public for the first time - in the Herald. We compare health boards, investigate where lives are being lost and the battle to save them.
* Today - District health boards compared, is death rate linked to healthcare quality, and how a simple check-list helps surgeons avoid mistakes.
* Tomorrow - Waitemata DHB boosts heart-care capacity. A bereaved father questions medical justice.
* Wednesday - Waikato DHB strives to understand its high death rate, medication safety, and a doctor's apology.
* Thursday - Palliative care helps Auckland DHB's good performance. A widow fights for changes.
* Friday - Obesity skews the statistics in South Auckland. Lives saved by reduction of blood infections.