Worldwide, two in three deaths - 35 million each year - are unregistered. Around 180 countries that are home to 80 per cent of the world's population do not collect reliable cause of death statistics, researchers said.
A team led by researchers at the University of Melbourne and the University of Washington redesigned a short 'verbal autopsy' questionnaire and tested it in India, the Philippines, Mexico, and Tanzania.
The app was then field-tested in China, Sri Lanka and Papua New Guinea.
"Without accurate cause of death information, we can't monitor disease and injury trends, we can't keep track of emerging health problems and we don't have any markers to show us whether programmes and policies are actually working," said Professor Alan Lopez from the University of Melbourne.
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"So if you live in a country where no-one is dying from malaria, then why are you pouring money into malaria-prevention programmes? And conversely, if people are dying from lung cancer, why aren't you investing in tobacco control?" said Lopez.
"Verbal autopsy research has shown that computer models are just as accurate as physicians in making diagnoses based on verbal autopsy data, at a fraction of the cost," said Christopher Murray, Director Institute for Health Metrics and Evaluation (IHME) at the University of Washington.
"In countries with scarce data on causes of death, policymakers need this information to better understand local disease burden and effectively allocate resources," said Murray.
Computers can reliably provide a diagnosis by linking symptoms with a specific cause of death in real-time. The instant provision of information overcomes what can be a 10-year lag between the death and the doctor's report.
"Relying on doctors to collect information about causes of death in rural populations is not helpful," Lopez said.
The research was published in the journal BMC Medicine.