Thousands of candidates, hundreds of parties, endless combinations of possible coalitions – spare a thought for India's pollsters, tasked with making sense of the country's fiendishly complicated politics ahead of a general election due by May.

The ruling Bharatiya Janata Party (BJP) led by Prime Minister Narendra Modi won a surprise majority in 2014. Until last year, many predicted a similar result. But amid rising anger over unemployment and a fall in rural incomes, the BJP lost key state elections in December, making this contest more closely fought than first expected.

That means surveys conducted on behalf of newspapers and TV channels will be closely scrutinised. Some of India's top pollsters, however, told Reuters current surveys could be wide of the mark until the parties finalise alliances, which could be as late as April – and even then, there are challenges.

"In India, there are certain relationships between caste, religion and allegiance," said VK Bajaj, chief executive of Today's Chanakya, the only polling firm to predict the BJP would win an outright majority in 2014. "We have to do checks and counter-checks when collecting our samples."

Chequered Past

Opinion polls grew in popularity in India in the 1990s, after economic liberalisation saw a boom in privately-owned newspapers and TV channels, all demanding their own surveys.

In 1998 and 1999, the polls closely predicted the share of seats for the winning BJP-led coalition, according to data collected by Praveen Rai, an analyst who has tracked opinion polls in India for more than 15 years at the Centre for the Study of Developing Societies, which also runs its own surveys.

But in the last three elections, polls have been significantly wide of the mark. In 2004 and 2009 the victorious Congress alliance was completely underestimated, while in 2014 only Bajaj's firm predicted the BJP would win an outright majority.

Elections in India have become "increasingly multi-varied", Rai said, with the emergence of regional parties complicating pollsters' efforts.

Reality On The Ground

Many polls are conducted face-to-face, and collecting representative samples can be hard in a country that still has several armed separatist movements and tribal communities unused to opinion polling.

When CNX, one of India's largest polling companies, conducts fieldwork in rural Chhattisgarh and Jharkhand – two states with large tribal populations – it often finds many are unfamiliar with the concept of opinion polls.

"In areas where people are not so educated it is difficult for them to understand," said Bhawesh Jha, CNX's founder.

Elsewhere, a lack of trust in why polls are conducted and how the data is used means respondents are also less truthful than other countries, pollsters said.

"Dubious opinion polls conducted by some media houses to sway the elections for political parties ... has definitely created a bad name for the polling industry in India," Rai said.

India lacks strong data protection laws like those in North America and Europe, and many people still believe their details will be passed on to political parties, Rai and Jha said, meaning answers were often those they think the pollster wants to hear.

"We have to convince people we are not going to reveal their identity," Jha said.

Complex Arithmetic

Current polls are making large assumptions, no more so than in Uttar Pradesh, India's largest state with a population of more than 200 million that accounts for nearly a fifth of the seats in India's lower house.

Results there have been so difficult to predict that the state has earned the nickname "Ulta Pradesh" - a play on the Hindi word meaning "reverse" - for its ability to confound experts.

A recent poll there found that if two regional parties already in alliance joined forced with the main opposition Congress, the BJP would be wiped out in the state, almost certainly losing power nationally.

But like other states in India, much depends on who contests from where – and to what extent Congress stands its candidates down to allow regional parties a run.

Until the final seats sharing agreements and candidate lists are announced – which may not be until April – current polls are little more than guesswork, said Today's Chanakya chief Bajaj.

"We have to wait until the final alliances come out," he said. "It is not possible to do anything until that."