Just because an animal hasn't been seen for a long time, doesn't mean it's extinct, according to statistical research that sheds new light on the plight of the dodo.

Dr David Roberts of the Royal Botanic Gardens, Kew in the U.K. and Dr Andrew Solow of the Woods Hole Oceanographic Institution, Massachusetts, in the U.S. estimate in today's issue of the journal Nature that the dodo actually became extinct nearly 30 years after its last reliable sighting.

Accurately estimating when a species becomes extinct is difficult as rare individuals can survive undetected for years after they are last seen. While the idea that an animal is not necessarily extinct just because it hasn't been seen for a while may seem obvious, extinction dates have traditionally been set as the date of last sighting.

To estimate the animal's actual point of extinction, Roberts and Solow applied a statistical test to the last 10 recorded dodo sightings. Their maths related the probability of an animal still existing at particular times after it was last seen.

While the extinction of the ungainly bird is commonly dated to 1662, the last time it was seen on an islet off Mauritius, Roberts and Solow's calculation put the date as 1690.

Australian biologist Professor Des Cooper of Macquarie University in Sydney said it would be interesting to test the technique on a number of Australian animals. Researchers have rediscovered some of these after they thought they were extinct.

He gave the example of the marsupial Gilbert's Potoroo which, up until 1994, was thought to have been extinct for 100 or 120 years.

"A graduate student from the University of Western Australia went down to somewhere in the south west of the state to try to look for brush-tailed bettongs and to her utter astonishment started trapping Gilbert's Potoroo. Every one was quite taken aback," Cooper told ABC Science Online.

He also says it could be interesting to apply the statistical test to the thylacine in Tasmania, the Tasmanian devil, the night parrot in Central Australia, and even the disappearance of the megafauna in Australia.

To work out the 'new' dodo extinction date the researchers used actual sightings of the animal but Cooper said that fossil data could be used as a substitute.

However he said he would "be inclined to raise my eyebrows" at the idea of applying the concept to the extinction of the dinosaurs because he did not think there would be accurate enough data.

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