The large jump in India’s tiger population which was announced by officials in Delhi last month was widely greeted as a positive – and hugely unexpected – piece of news.

However, doubts have now been cast upon the methodology used to determine whether the subcontinent’s most famous big cat is indeed enjoying a surge in numbers.

A combined team of researchers from the University of Oxford, the Wildlife Conservation Society (WCS) and the Indian Statistical Institute (ISI) has analysed the process involved in the collation of the data – and suggested that it is likely to have caused “irreproducible and inaccurate results”.

On January 20, the Indian government outlined the results of its latest tiger census, with the happy revelation that the country’s tiger head count has apparently soared by 30 per cent during the last four years – from 1,706 in 2010 to 2,226 at the end of 2014.

India is home to some 70 per cent of the world’s tigers, and the figures have been hailed as a remarkable sign of progress in the bid to protect this endangered species – described as “a huge success story” by Environment Minister Prakash Javadekar.

“Never before has such an exercise been taken on such a massive scale where we have unique photographs of 80 per cent of India’s tigers,” Mr Javadekar explained. “While the tiger population is falling in the world, it is rising in India. This is great news.”

The new questions about these results are focused on the main procedure used to assess the size of India’s tiger population – the concept of ‘index-calibration’.

In doubt: The counting method behind the new results has been questioned (Photo: AFP)

This method relies upon the taking of accurate measurements in one small area – and then extrapolating them over a wider field to obtain a bigger picture.

In the case of tiger numbers, this means deploying a precise way of counting – such as camera traps, triggered by movement, which obtain photographs of individual animals – in a closely defined place, then stretching the numbers to a larger region by using secondary evidence – such as paw prints and tracks – to support the findings.

This process is often used in conservation programmes – partly because, although it produces solid results, camera trapping is both expensive and time consuming, making its use over a bigger space financially impractical.

As a consequence, data produced – says the team from the University of Oxford – can be skewed.

Arjun Gopalaswamy, of the Wildlife Conservation Research Unit at the university’s Department of Zoology, said the latest Indian figures may be “compromised”.

“Our study shows that index-calibration models are so fragile that even a 10 per cent uncertainty in detection rates severely compromises what we can reliably infer from them,” he argued.

“Our empirical test with data from the Indian tiger survey proved that such calibrations yield irreproducible and inaccurate results.”

The process also involves mathematical leaps of faith, Mr Gopalaswamy added.

“Index-calibration relies on the assumption that detection rates of animal evidence are high and unvarying,” he said.

“In reality, this is nearly impossible to achieve.”

Endangered: Illegal tiger skins seized in northern India in 2001 (Photo: EPA)

His comments were echoed by his departmental colleague Professor David Macdonald, who said that a greater focus on accuracy is needed when assessing the numerical strengths or weaknesses of elusive creatures such as the tiger.

“Index-calibration can work well if the correlations are tight and consistent,” he said.

“But often they aren’t, and many of us – myself included, for example, in the context of estimating numbers of mink and water voles in the UK – have been using the technique without appreciating its risks.

“Our intention is to help conservationists by highlighting the conditions when index-calibration can be misleading.

“Everybody will benefit from greater accuracy when it comes to counting rare animals.”

The findings have been published in the research journal Methods in Ecology and Evolution.

However, the team has also underlined that while the size of the perceived boom in Indian tiger numbers may not be backed up by reliable statistics, this does not mean that an upsurge has not occurred.

“We are not at all disputing that tiger numbers have increased in many locations in India in the last eight years,” said Dr Ullas Karanth of the WCS.

“But the method employed to measure this increase is not sufficiently robust to measure changes at regional and country-wide levels.”

India’s tiger population plummeted during the 20th century as a result of poaching, indiscriminate hunting and the belief in certain areas of the Far East, notably China, that the big cat’s body parts have medicinal and aphrodisiacal properties.

Trading in tiger parts has been illegal since 1973 under the Convention on International Trade in Endangered Species of Wild Fauna and Flora.

Trafficking in tiger parts has officially been banned in China since 1993 – although there is a flourishing black market which continues to threaten the creature’s survival.