Archive for the 'Epistemology' Category

Today’s community service announcement …

… comes from the language of scientific (well, economic) argument.

The phrase “X is consistent with Y” is actually a very, very weak statement.  All it’s saying is that X doesn’t provide evidence against Y.  Here’s a handy flow chart:

X is … Y:    “consistent with” < “suggestive of” < “evidence for” < “proof of”

Political comic strips around the Mississippi Bubble of the 1710s

I wish that I had time to read this paper by David Levy and Sandra Peart.

It’s about political comics (cartoons) drawn to depict John Law and the Mississippi Bubble of the early 1700s.  It also speaks to subtlely different meanings of the words “alchemy” and “occult” than we are used to today. Here is an early paragraph in the paper:

Non-transparency induces a hierarchy of knowledge. The most extreme form of that sort of hierarchy might be called the cult of expertise in which expertise is said to be accompanied by godlike powers, the ability to unbind scarcity of matter and time. The earliest debates over hierarchy focused on whether such claims are credible or not.

Here is the abstract:

Economists have occasionally noticed the appearance of economists in cartoons produced for public amusement during crises. Yet the message behind such images has been less than fully appreciated. This paper provides evidence of such inattention in the context of the eighteenth century speculation known as the Mississippi Bubble. A cartoon in The Great Mirror of Folly imagines John Law in a cart that flies through the air drawn by a pair of beasts, reportedly chickens. The cart is not drawn by chickens, however, but by a Biblical beast whose forefather spoke to Eve about the consequences of eating from the tree of the knowledge. The religious image signifies the danger associated with knowledge. The paper thus demonstrates how images of the Mississippi Bubble focused on the hierarchy of knowledge induced by non-transparency. Many of the images show madness caused by alchemy, the hidden or “occult.”

Hat tip: Tyler Cowen.

Double-yolk eggs, clustering and the financial crisis

I happened to be listening when Radio 4’s “Today Show” had a little debate about the probability of getting a pack of six double-yolk eggs.  Tim Harford, who they called to help them sort it out, relates the story here.

So there are two thinking styles here. One is to solve the probability problem as posed. The other is to apply some common sense to figure out whether the probability problem makes any sense. We need both. Common sense can be misleading, but so can precise-sounding misspecifications of real world problems.

There are lessons here for the credit crunch. When the quants calculate that Goldman Sachs had seen 25 standard deviation events, several days in a row, we must conclude not that Goldman Sachs was unlucky, but that the models weren’t accurate depictions of reality.

One listener later solved the two-yolk problem. Apparently workers in egg-packing plants sort out twin-yolk eggs for themselves. If there are too many, they pack the leftovers into cartons. In other words, twin-yolk eggs cluster together. No wonder so many Today listeners have experienced bountiful cartons.

Mortgage backed securities experienced clustered losses in much the same unexpected way. If only more bankers had pondered the fable of the eggs.

The link Tim gives in the middle of my quote is to this piece, also by Tim, at the FT.  Here’s the bit that Tim is referring to (emphasis at the end is mine):

What really screws up a forecast is a “structural break”, which means that some underlying parameter has changed in a way that wasn’t anticipated in the forecaster’s model.

These breaks happen with alarming frequency, but the real problem is that conventional forecasting approaches do not recognise them even after they have happened. [Snip some examples]

In all these cases, the forecasts were wrong because they had an inbuilt view of the “equilibrium” … In each case, the equilibrium changed to something new, and in each case, the forecasters wrongly predicted a return to business as usual, again and again. The lesson is that a forecasting technique that cannot deal with structural breaks is a forecasting technique that can misfire almost indefinitely.

Hendry’s ultimate goal is to forecast structural breaks. That is almost impossible: it requires a parallel model (or models) of external forces – anything from a technological breakthrough to a legislative change to a war.

Some of these structural breaks will never be predictable, although Hendry believes forecasters can and should do more to try to anticipate them.

But even if structural breaks cannot be predicted, that is no excuse for nihilism. Hendry’s methodology has already produced something worth having: the ability to spot structural breaks as they are happening. Even if Hendry cannot predict when the world will change, his computer-automated techniques can quickly spot the change after the fact.

That might sound pointless.

In fact, given that traditional economic forecasts miss structural breaks all the time, it is both difficult to achieve and useful.

Talking to Hendry, I was reminded of one of the most famous laments to be heard when the credit crisis broke in the summer. “We were seeing things that were 25-standard deviation moves, several days in a row,” said Goldman Sachs’ chief financial officer. One day should have been enough to realise that the world had changed.

That’s pretty hard-core.  Imagine if under your maintained hypothesis, what just happened was a 25-standard deviation event.  That’s a “holy fuck” moment.  David Viniar, the GS CFO, then suggests that they occurred for several days in a row.  A variety of people (for example, Brad DeLong, Felix Salmon and Chris Dillow) have pointed out that a 25-standard deviation event is so staggeringly unlikely that the universe isn’t old enough for us to seriously believe that one has ever occurred.  It is therefore absurd to propose that even a single such event occurred.   The idea that several of them happened in the space of a few days is beyond imagining.

Which is why Tim Harford pointed out that even after the first day where, according to their models, it appeared as though a 25-standard deviation event had just occurred, it should have been obvious to anyone with the slightest understanding of probability and statistics that they were staring at a structural break.

In particular, as we now know, asset returns have thicker tails than previously thought and, possibly more importantly, the correlation of asset returns varies with the magnitude of that return.  For exceptionally bad outcomes, asset returns are significantly correlated.

Epistemology in the social sciences (economics included)

I’m not sure how I came across it, but Daniel Little has a post summarising a 2006 article by Gabriel Abend:  “Styles of Sociological Thought: Sociologies, Epistemologies, and the Mexican and U.S. Quests for Truth“.  Daniel writes:

Abend attempts to take the measure of a particularly profound form of difference that might be postulated within the domain of world sociology: the idea that different national traditions of sociology may embody different epistemological frameworks that make their results genuinely incommensurable.


Consider this tabulation of results on the question of the role of evidence and theory taken by the two sets of articles:


Here is a striking tabulation of epistemic differences between the two samples:

Abend believes that these basic epistemological differences between U.S. and Mexican sociology imply a fundamental incommensurability of results:

To consider the epistemological thesis, let us pose the following thought experiment. Suppose a Mexican sociologist claims p and a U.S. sociologist claims not-p.  Carnap’s or Popper’s epistemology would have the empirical world arbitrate between these two theoretical claims. But, as we have seen, sociologists in Mexico and the United States hold different stances regarding what a theory should be, what an explanation should look like, what rules of inference and standards of proof should be stipulated, what role evidence should play, and so on. The empirical world could only adjudicate the dispute if an agreement on these epistemological presuppositions could be reached (and there are good reasons to expect that in such a situation neither side would be willing to give up its epistemology). Furthermore, it seems to me that my thought experiment to some degree misses the point. For it imagines a situation in which a Mexican sociologist claims p and a U.S. sociologist claims not-p, failing to realize that that would only be possible if the problem were articulated in similar terms. However, we have seen that Mexican and U.S. sociologies also differ in how problems are articulated—rather than p and not-p, one should probably speak of p and q.  I believe that Mexican and U.S. sociologies are perceptually and semantically incommensurable as well. (27)

Though Abend’s analysis is comparative, I find his analysis of the epistemological assumptions underlying the U.S. cases to be highly insightful all by itself.  In just a few pages he captures what seem to me to be the core epistemological assumptions of the conduct of sociological research in the U.S.  These include:

  • the assumption of “general regular reality” (the assumption that social phenomena are “really” governed by underlying regularities)
  • deductivism
  • epistemic objectivity
  • a preference for quantification and abstract vocabulary
  • separation of fact and value; value neutrality

There is a clear (?) parallel dispute in the study of economics as well, made all the more complicated by the allegations leveled at economics as a discipline as a result of the global financial crisis.

Nassim Taleb takes bat, ball; goes home

The author of The Black Swan doesn’t approve of the looming reappointment of Ben Bernanke as chairman of the US Federal Reserve.  Writing in the Huffington Post, he says:

What I am seeing and hearing on the news — the reappointment of Bernanke — is too hard for me to bear. I cannot believe that we, in the 21st century, can accept living in such a society. I am not blaming Bernanke (he doesn’t even know he doesn’t understand how things work or that the tools he uses are not empirical); it is the Senators appointing him who are totally irresponsible — as if we promoted every doctor who caused malpractice. The world has never, never been as fragile. Economics make[sic] homeopath and alternative healers look empirical and scientific.

No news, no press, no Davos, no suit-and-tie fraudsters, no fools. I need to withdraw as immediately as possible into the Platonic quiet of my library, work on my next book, find solace in science and philosophy, and mull the next step. I will also structure trades with my Universa friends to bet on the next mistake by Bernanke, Summers, and Geithner. I will only (briefly) emerge from my hiatus when the publishers force me to do so upon the publication of the paperback edition of The Black Swan.


That’s quite a god complex Taleb’s got going on there (“he doesn’t even know he doesn’t understand how things work”).