People don't decide their position on BREXIT by looking at the numbers: they choose their position and then seek numbers to justify it. Most of the numbers quoted on either side can't be trusted. This is corrupting public discourse by damaging the credibility of numbers that do tell a clear story.
The ferocity of debate in politics is often inversely proportional to the amount of actual hard evidence available on the topic. Whether the UK should stay in the EU is a typical example. Many decide they don't like the loss of freedom required when you are a member of the club and call for exit; others, like me, decide that the compromises required and the bureaucratic overhang of membership are worth it because the gains from collaboration are worth it. Some choices are more atavistic: many presume (probably incorrectly) that uncontrolled immigration is caused by EU membership (and they also believe the populist myth that immigrants are the cause of many other problems in society). Neither side reaches their conclusion because they have done some calculations: the conclusions come from deep emotional value choices not statistics. But the debate pretends otherwise and dredges up volumes of statistics to confirm the emotionally reached decision.
The numbers thrown around in the debate are proxies for that emotional choice. Most people can't admit that they didn't reason their way to their choice and they seek what look like the rational arguments that got them there. But this process suffers from all of the cognitive biases that so beset much thinking (especially confirmation bias where people are more likely to believe numbers supporting the position they already hold). People seek the numbers that confirm their side of the debate.
Nothing illustrates this better than the meme of European bureaucracy. Even EU supporters often complain about bureaucracy and many even quoted this tweet as an example:
Luckily for rational thinkers the BBC's More or Less programme (see this BBC article) decided to test the assertion. Their conclusion: there are no EU regulations specifically about cabbage sales and the 26,911 number originated in a complaint in the post-war USA about government regulations (and was probably mythical even then). Amazingly the same number of words has been repeated by many other objectors to bureaucracy without ever being validated in even the most cursory way against real documents.
The meme of EU bureaucracy is so strong that even pro-EU campaigners didn't think to challenge the basic facts in the assertion.
My main point, though, is about the harder economic numbers bandied about by the two sides and the extent to which they corrupt public discourse when we are debating significant issues. Both sides in the debate, for example, agree that leaving the EU would be disruptive. I agree: that is one of the few things that is clear. But there is a lot of disagreement about how disruptive leaving would be and even more on the economic benefits of staying or leaving. The leave campaign tend to assume that new trade treaties would be easy to negotiate quickly (not that there is any evidence to support this) and they assume that the benefits of an independent UK would be large when freed from the dead hand of EU regulation. The stay camp assume that renegotiation is a long and costly process and that many current jobs in the UK that depend on EU trade would be lost.
Both sides quote specific estimates for the economic benefit of their position. This is where the problem comes. Although specific numbers are quoted, the estimates have no credibility. John Kay, the economist and FT columnist addressed some of the problems with these kinds of estimates in a column in 2011. His concerns, though originally made about the economic rationale for big government projects, apply equally to any analysis of the case for staying or leaving:
...Because so many inputs to the analysis are invented, they can be chosen with a view to the desired result...
...The only information exercises such as these convey is the limits of the imagination of the people who have undertaken them….
...Yet the mistaken belief persists that these procedures provide an objective basis for decision making...
...We do great damage by claiming to know things that are not known, by asserting certainty in the face of uncertainty and ambiguity, and by attaching a veneer of rationality to decisions that have in fact been made on other, rarely articulated, grounds. The paradoxical result is all too obvious. The public sector and large bureaucratic organisations appear as paragons of good decision making process and exemplars of bad decisions.
I would add an extra concern. The specific nature of the forecasts made by each side damage the credibility of almost any analysis intended to illuminate a major public decision. This is bad because, for example, when we have to decide how much to spend to mitigate global warming, the numbers we are shown will have no credibility and we will, most likely, make poor decisions as a result. Sometimes the numbers do point one way on a major decision. If we pollute public discourse with spurious, over-precise analysis, we will undermine our ability to make good decisions when the evidence is clear.
When deciding whether to stay in the EU or to invest in major infrastructure projects we can never have precise estimates of the costs and benefits. We should not deny the uncertainty by pretending that we do. We can't decide on whether to stay in the EU by purely objective analysis: the uncertainties are too large. We should admit that our choice is based on values not numbers. I believe that international cooperation is a better way to conduct our affairs than standing independent and alone, despite the cost in bureaucracy. I don't pretend I can prove it with economic models.
But, if you care about honesty in public debate, there is something you can do whatever position you hold on the EU. Donate to Full Fact's campaign to fact-check the debate. They don't care how you vote but they do care whether the debate is conducted honestly.