Exit polls: Do they get it right?

Saturday 27 February 2016 21.18
Unlike polls during the campaign, exit polls report what people say they did rather than what they will do
Unlike polls during the campaign, exit polls report what people say they did rather than what they will do

People tuning into RTÉ radio and television this morning are likely to hear lots about the exit poll.

Of course it will be at least tomorrow evening before the final result is known but the exit poll provides an update on this week’s final campaign polls and gives us an indication of what the early tallies will show through the morning.

Will it be a good guide to real-life events?

Exit polls certainly should be pretty accurate. Unlike polls in the campaign, they report what people say they really did, not what they said they intended to do – which, as we all know, are not always as close to one another as we might like them to be.

Ballot box at the RDS, Dublin

It avoids the problems caused by late changes of mind, and by some respondents simply not turning out.

The exit poll also has a much larger sample – typically 3,000 or more – and takes its respondents from more locations than other face-to-face polls.

Looking at the four general election exit polls to date - all four of which were conducted by Lansdowne Market Research, now Millward Brown - the answer is yes, they are very good guides.

You can see the details of the four exit polls to date below, matched against the final result and it is clear that poll figures and actual results match up very well indeed.

The best way to think about how close polls and results are to one another is to think about how many people we would have to move from one party to another to get the poll figures and the results matching perfectly.

Last time, in 2011, we would have to move only 28 in every thousand polled: just under 3% of respondents. The poll could be considered 97% correct.

The main error was in the FF vote share, which was underestimated by just over 2%, while Independents were overestimated by 1.5%.

Move those into the FF column, together with most of the 1.1% overestimation of the Labour vote and the match is almost exact.

In each of the other three cases, the polls were also very accurate. In 1997, the poll was 96% correct: we would have had to move 36 in every thousand to get the right result.

A voter casts her vote in Ballinlough, Co Cork

Because Fianna Fáil was overestimated and Fine Gael underestimated, the gap between the two was 3.5% closer in reality than in the polls.

This might have made a significant difference in a UK-style first-past-the-post election but makes a smaller difference here, although with Fianna Fáil in touching distance of winning a majority of seats, then small overestimates and underestimates can be important.

In 2002 – when the earlier polls were very wide of the mark – the exit was again a very accurate guide; again just 26 in every 1000 were "misplaced".

The biggest error was a 1.9 overestimation of the Fine Gael vote – the reverse of the error in 1997. This time the Fianna Fáil estimate was just 0.5% out.

Finally in 2007, the exit poll was the best we have seen, with only 13 in every 1000 having to move to get the right result. This is close to 99% right.

If the exit poll on Saturday can do better than that I shall be very happy, and so will Behaviour and Attitudes.

2011

Poll

Result

Difference

FF

15.1

17.4

-2.4

FG

36.1

36.1

0.0

Labour

20.5

19.4

1.1

Green

2.7

2.7

0.0

SF

10.1

9.9

0.2

Ind

14.2

12.7

1.5

Others

1.3

1.8

-0.5

       

1997

Votes

result

Difference

FF

40.89

39.3

1.6

FG

26.08

28.0

-1.9

Labour

11.32

10.4

0.9

PD

4.579

4.7

-0.1

DL

2.415

2.5

-0.1

Green

3.473

2.8

0.7

SF

2.881

2.5

0.4

Others

8.365

9.8

-1.4

       

2002

Poll

Result

Difference

FF

42.0

41.5

0.5

FG

20.6

22.5

-1.9

Labour

12.5

10.8

1.7

PD

3.5

4

-0.5

Green

4.6

3.8

0.8

SF

7.2

6.5

0.7

Others

9.7

10.9

-1.2

       

2007

Poll

Result

Difference

FF

41.62

41.6

0.0

FG

26.29

27.3

-1.0

Lab

9.933

10.1

-0.2

PD

2.601

2.7

-0.1

Green

4.823

4.7

0.1

SF

7.264

6.9

0.4

Ind

7.5

6.6

0.9

This Election 2016 RTÉ exit poll was conducted by Behaviour and Attitudes for RTÉ in partnership with the School of Politics and International Relations of UCD, The Department of Government in UCC, The School of Politics, International Studies and Philosophy in Queen's University Belfast, and Trinity College Dublin.


By Michael Marsh, Emeritus Professor, Trinity College Dublin