Bihar Exit Polls: How reliable are exit polls? The answer lies in sample size

Hyderabad: On 7 November, the exit polls by many agencies showed that the opposition coalition - Mahagathbandhan led by Tejashwi Yadav's Rashtriya Janta Dal (RJD) – will beat Bihar Chief Minister Nitish Kumar-led NDA alliance to win the Bihar Assembly elections.

Saturday's Chanakya exit poll showed Mahagathbandhan bagging 180 seats while Dainik Bhaskar showed Mahagathbandhan winning only around 80 seats.

So, how does these exit polls work and how reliable are they in predicting which parties or leaders will people support during elections?

Exit polls, at a basic level, are the results of surveys conducted on voters after they have cast their votes. These exit polls are conducted by various research and media organisations. They surveys can be a questionnaire filled in person or done over phone, as some research organisations do.

The most important point to remember while banking on any exit poll is its sample size. In simple terms, a sample size is the number of people who participated in the exit polls. Different agencies have differernt sample sizes. For example, India Today-Axis My India exit poll claimed that they derived their results from contacting 63,000 persons, which means 250 participants per Assembly seat. Similarly, India Ahead-ETG exit poll claimed to have used 400 samples per Assembly seat for their research.

The two most common sampling methods used by any research agency for exit polls are random sampling and systematic sampling. In random sampling, voters are picked at random and asked about their voting preferences. In systematic sampling, voters are selected beforehand based on their age, gender, caste, religion and so on. Research agencies prefer random sampling. Since the population of Bihar is very diverse, the methodology behind sampling affects the outcome.

The methodology behind the sampling is important. For example, caste is an important factor when it comes to voting. Some of agencies have made this their priority while analysing data. For example, In India Today-Axis My India exit poll, both NDA and Mahagathbandhan were shown to have won 43 per cent of voters in the Mithilanchal region. Mithilanchal region is in northern Bihar where people speak Maithili. Also, this region is a base for upper caste communities who prefer voting for the NDA. Though, both alliances have got the same percentage of votes, the exit poll said NDA will get only 23 votes, while Mahagathbandhan will get 36 votes. Also, Axis My India exit polls stated that, "In Darbhanga and Sitamarhi, where the upper caste communities have the highest population, more votes would go to NDA, while people in Mithilanchal would vote more for Mahagathbandhan. The vote gap between NDA and Mahagathbandhan in Darbhanga and Sitamarhi is too high."

The reliability of any exit poll depends on the number of samples and the demographic category they have chosen during the survey.

The party or alliance which will win 122 seats in 243-seat Bihar Assembly will form the government.

Most of the exit polls conducted by different agencies in Bihar have given advantage to Mahagathbandhan. The India Today-Axis My India exit poll predicts Mahagathbandhan (comprising RJD, Congress, CPI, CPI-M and CPI-ML) will win 139-161 seats, while the NDA consisting of JD(U), BJP, HAM and VIP will win 69-91 seats. Chirag Paswan-led Lok Janshakti Party (LJP) will win three to five seats while other parties/alliance will win six to eight seats.

Chanakya, the research agency who rightly predicted in 2014 and 2019 that BJP will win above 280 seats in 2014 and above 300 seats in 2019 in both Lok Sabha elections, has predicted that Mahagathbandhan will have a massive victory with above 180 seats and NDA alliance will only get 55 seats.

Some of the exit polls also predicted a tight fight between both the alliances. Times Now- C Voter gave 120 seats to Mahagathbandan and 116 seats to the NDA alliance. LJP is projected to win one seat in this exit poll.

Republic TV- Jan Ki Baat gave Mahagathbandhan a lead of 118 to 138 seats while giving the ruling alliance 91-117 seats and LJP five to eight seats.

Dainik Bhaskar, a newspaper based in north India, is the only agency who gave a majority to the NDA alliance in its exit polls. They gave 120-127 seats to NDA alliance, 71-81 seats to Mahagathbandhan, 12-13 seats to LJP, and 19-27 seats to others.

The exit polls are not the final results. They are just a collection of surveys of less than one per cent of the population which are then used to find the trends over an entire state or country.

The counting of votes for Bihar Assembly elections is on November 10.