Like any exit polls, those in India are imperfect, but their accuracy has improved in recent years.
The major exit polls in 2014 correctly predicted a win for the B.J.P.-led coalition. In 2009, exit polls accurately forecast that Congress would gain the most seats, though the data underestimated the number ultimately won by the Congress alliance.
The current exit polls were conducted by Indian research and survey organizations, many with decades of experience, which partnered with news media outlets. According to the polling organizations, the sample size for each exit poll varied from 40,000 voters to two million.
Some analysts have cautioned that exit polls may overstate Mr. Modi’s support because some people would be scared to say they voted against him. But overall, political scientists said they seemed reliable.
“In the majority of the cases, exit polls have depicted the true picture,” said Josukutty Cheriantharayil Abraham, director of the survey research center at the University of Kerala. “It may not be correct in terms of the number of seats or vote percentage, but it could definitely show the trends.”