Opinion polling on Brexit has not necessarily been the best. Highly politically contentious issues do tend to attract polling that is sub-optimal, and Brexit has followed that trend. I’ve seen several Brexit polls coming up with surprising findings based on agree/disagree statements – that is, questions asked in the form:

Do you agree with the following statement? I think Brexit is great

Agree

Disagree

Don’t know

This is a very common way of asking questions, but one that has a lot of problems. One of the basic rules in writing fair and balanced survey questions is that you should try to given equal prominence to both sides of the argument. Rather than ask “Do you support X?”, a survey should ask “Do you support or oppose X?”. In practice agree-disagree statements break that basic rule – they ask people whether they agree/disagree with one side of the argument, without mentioning the other side of the argument.

In some cases the opposite side of the argument is implicit. If the statement is “Theresa May is doing a good job”, then it is obvious to most respondents that the alternative view is that May is doing a bad job (or perhaps an average job). Even when it’s as obvious as this it still sometimes to make a difference – for whatever reason, decades of academic research into questionnaire design suggest people are more likely to agree with statements than to disagree with them, regardless of what the statement is (generally referred to as “acquiescence bias”).

There is a substantial body of academic evidence exploring this phenomenon (see, for example Schuman & Presser in the 1980s, or the recent work of Jon Krosnick) it tends to find around 10%-20% of people will agree with both a statement and its opposite, if it is asked in both directions. Various explanations have been put forward for this in academic studies – that it’s a result of personality type, or that it is satisficing (people just trying to get through a survey with minimal effort). The point is that it exists.

This is not just a theoretical issue that turns up in artificial academic experiments – they are plenty of real life examples in published polls. My favourite remains this ComRes poll for UKIP back in 2009. It asked if people agreed or disagreed with a number of statements including “Britain should remain a full member of the EU” and “Britain should leave the European Union but maintain close trading links”. 55% of people agreed that Britain should remain a full member of the EU. 55% of people also agreed that Britain should leave the EU. In other words, at least 10% of the same respondents agreed both that Britain should remain AND leave.

There is another good real life example in this poll. 42% agreed with a statement saying that “divorce should not be made too easy, so as to encourage couples to stay together”. However, 69% of the same sample also agreed that divorce should be “as quick and easy as possible”. At least 11% of the sample agreed both that divorce should be as easy as possible AND that it should not be too easy.

Examples like this of polls that asked both sides of the argument and produced contradictory findings are interesting quirks – but since they asked the statement in both directions they don’t mislead. However, it is easy to imagine how they would risk being misleading if they had asked the statement in only one direction. If that poll had only asked the pro-Brexit statement, then it would have looked as if a majority supported leaving. If the poll had only asked the anti-Leave statement, then it would have looked as if a majority supported staying. With agree-disagree statements, if you don’t ask both sides, you risk getting a very skewed picture.

In practice, I fear the problem is often far more serious in published political polls. The academic studies tend to use quite neutrally worded, simple, straightforward statements. In the sort of political polling for pressure groups and campaigning groups that you see in real life the statements are often far more forcefully worded, and are often statements that justify or promote an opinion – below are some examples I’ve seen asked as agree-disagree statements in polls:

“The Brexit process has gone on long enough so MPs should back the Prime Minister’s deal and get it done”

“The result of the 2016 Referendum should be respected and there should be no second referendum”

“The government must enforce the minimum wage so we have a level playing field and employers can’t squeeze out British workers by employing immigrants on the cheap”

I don’t pick these because they are particularly bad (I’ve seen much worse), only to illustrate the difference. These are statements that are making an active argument in favour of an opinion, where the argument in the opposite direction is not being made. They do not give a reason why MPs may not want to back the Prime Minister’s deal, why a second referendum might be a good idea, why enforcing the minimum wage might be bad. It is easy to imagine that respondents might find these statements convincing… but that they might have found the opposite opinion just as convincing if they’d been presented with that. I would expect questions like this to produce a much larger bias in the direction of the statement if asked as an agree-disagree statement.

With a few exceptions I normally try to avoid running agree-disagree statements, but we ran some specially to illustrate the problems, splitting the sample so that one group of respondents were asked if they agreed or disagreed with a statement, and a second group where asked if they agreed-disagreed with a contrasting statement. As expected, it produces varied results.

For simple questions, like whether Theresa May is doing a good job, the difference is small (people disagreed with the statement that “Theresa May is doing a good job by 57% to 15% and agreed with the statement that “Theresa May is doing a bad job” by 52% to 18%. Almost a mirror image. On some of the other questions, the differences were stark:

If you asked if people agree that “The NHS needs reform more than it needs extra money” then people agree by 43% to 23%. However, if you ask if people agree with the opposite statement, that “The NHS needs extra money more than it needs reform”, then people also agree, by 53% to 20%.

If you ask if people agree or disagree that “NHS services should be tailored to the needs of populations in local areas, even if this means that there are differences across the country as a whole” than people agree by 43% to 18%. However, if you ask if they agree or disagree with a statement putting the opposite opinion – “NHS services should be the same across the country” – then people agree by 88% to 2%!

By 67% to 12% people agree with the statement that “Brexit is the most important issue facing the government and should be its top priority”. However, by 44% to 26% they also agree with the statement “There are more important issues that the government should be dealing with than Brexit”

I could go on – there are more results here (summary, full tabs) – but I hope the point is made. Agree/disagree statements appear to produce a consistent bias in favour of the statement, and while this can be minor in questions asking simple statements of opinion, if the statements amount to political arguments the scale of the bias can be huge.

A common suggested solution to this issue is to make sure that the statements in a survey are balanced, with an equal amount of statements in each direction. So, for example, if you were doing a survey about attitudes towards higher taxes, rather than asking people if they agreed or disagreed with ten statements in favour of high taxes, you’d ask if people agreed or disagreed with five statements in favour of higher taxes and five statements in favour of lower taxes.

This is certainly an improvement, but is still less than ideal. First it can produce contradictory results like the examples above. Secondly, in practice it can often result in some rather artificial and clunky sounding questions and double-negatives. Finally, in practice it is often difficult to make sure statements really are balanced (too often I have seen surveys that attempt a balanced statement grid, but where the statements in one direction are hard-hitting and compelling, and in the other direction are deliberately soft-balled or unappetising).

The better solution is not to ask them as agree-disagree statements at all. Change them into questions with specific answers – instead of asking if people agree that “Theresa May is going a good job”, ask if May is doing a good or bad job. Instead of asking if people agree that “The NHS needs reform more than it needs more money”, ask what people think the NHS needs more – reform or more money? Questions like the examples I gave above can easily be made better by pairing the contrasting statements, and asking which better reflects respondents views:

Asked to pick between the two statements on NHS reform or funding, 41% of people think it needs reform more, 43% think it needs extra money more.

Asked to pick between the two statements on NHS services, 36% think they should be tailored to local areas, 52% would prefer them to be the same across the whole country.

Asked to pick between the two statements on the importance of Brexit, 58% think it is the most important issue facing the government, 27% think there are more important issues the government should be dealing with instead.

So what does this mean when it comes to interpreting real polls?

The sad truth is that, despite the known problems with agree-disagree statements, they are far from uncommon. They are quick to ask, require almost no effort at all to script and are very easy for clients after a quick headline to interpret. And I fear there are some clients to whom the problems with bias are an advantage, not a obstacle; you often see them in polls commissioned by campaigning groups and pressure groups with a clear interest in getting a particular result.

Whenever judging a poll (and this goes to observers reading them, and journalists choosing whether to report them) my advice has always been to go to polling companies websites and look at the data tables – look at the actual numbers and the actual question wording. If the questions behind the headlines have been asked using agree-disagree statements, you should be sceptical. It’s a structure that does have an inherent bias, and does result in more people agreeing than if the question had been asked a different way.

Consider how the results may have been very different if the statement had been asked in the opposite direction. If it’s a good poll, you shouldn’t have to imagine that – the company should have made the effort to balance the poll by asking some of the statements in the opposite direction. If they haven’t made that effort, well, to me that rings some alarm bells.

If you get a poll that’s largely made up of agree-disagree statements, that are all worded in the direction that the client wants the respondent to answer rather than some in each direction, that use emotive and persuasive phrasing rather than bland and neutral wording? You would be right to be cautious.