Yogendra Yadav, quoted in ibnlive.com, explaining why the pollsters went horribly wrong in the UP elections.
"There are three reasons why the polls were so off the mark. First, there was a sampling error, especially in the exit polls. In an exit poll, you don’t choose the person you wish to interview. The voter chooses to walk to or walk away from the investigator stationed outside the polling station. Unless systematic precautions are taken, chances are that any exit poll will over-represent the well-off and upper caste and under-represent the poor and lower caste. This has resulted in systematic under-estimation of the BSP and over-estimation of the BJP over the last decade.
Secondly, there was a response bias. Those who voted for the BSP were less likely to say so to an outsider. Non-dalits who voted for the BSP may have been unwilling to admit it to themselves. This very unusual situation led to an over-reporting for the SP and BJP.
Finally, the vote-seats equation in UP turned out to be very skewed this time. For every one per cent of its votes, the BSP won 6.8 seats, compared to 3.8 for the SP and 3.0 for the BJP. This meant that the BSP won a large number of seats with very small margins. Even if you could foresee the exact vote share of the BSP, it was difficult to forecast the number of its seats."
First, sampling error. Second, response bias. Third, vote-seat equation skew.
And silly me. I thought these were the parameters pollsters took care of.
At least CNN IBN has admitted that they goofed. No one else has, till now.