DSGEs don't define priors!

I cannot think of a more difficult problem than forcing economic data to confirm economic models. Most economists have effecting ceded this and given into some effective mantle of bayesianism.

Hence the rise of DSGEs and their use as a means of forming a prior to a Bayesian VAR model.

Rather than to use MLE which is known to be unwieldy when fitting nonlinearly constrained VARs, economists now use the underlying model the DSGE to define a prior. (And so do the finance types, eg Ang-Piazzessi and the largely useless machinery-in the context of data fitting- of affine models for no arbitrage restrictions). 

Now let us be clear a DSGE can in no way define a prior. As an equation it can at best define a manifold in parameter space. This can be the max likelihood surface or the effective mode of this distribution but it cannot tell you the metric or any more about the actual prior distribution.

So how do economists even begin to think of using DSGEs to form priors for their VARs?

Go figure.

I will post links to books/papers on topic later on. I am certain that in no way can it truly make complete sense to anyone. (unless we can determine in more detail--flesh out---the notion of informative and noninformative "directions" in parameter spaces....even this will be challenging in that it will be easier to have informative/noninformative subspaces rather than manifolds!).

Why must we have priors?

Letter to Andrew Gelman

Andrew
While I am absolutely sympathetic to the Bayesian agenda I am often troubled by the requirement of having priors. We must have priors on the parameter of an infinite number of model we have never seen before and I find this troubling. There is a similarly troubling problem in economics of utility theory. Utility is on consumables. To be complete a consumer must assign utility to all sorts of things they never would have encountered. More recent versions of utility theory instead make consumption goods a portfolio of attributes. Cadillacs are x many units of luxury y of transport etc etc. And we can automatically have personal utilities to all these attributes.  

I don't ever see parameters. Some model have few and some have hundreds. Instead, I see data. So I don't know how to have an opinion on parameters themselves. Rather I think it far more natural to have opinions on the behavior of models. The prior predictive density is a good and sensible notion. Also if we has conditional densities for VARs then the prior conditional density. You have opinions about how variables interact and the forecast of some subset conditioning on the remainder.  That this may or may not give enough info to ascribe a proper prior in parameter space all the better. To the extent it does not we must arbitrarily pick one (eg reference prior or maxent prior subject to the data/model prior constraints).  Without reference to actual data I do not see much point in
trying to have any opinion at all. 

Please do let me know your thoughts. 

Best
Nick

Nick Firoozye

Why aren't there more Bayesian Econometricians?

From a letter to Brian Caplan and ccing Andrew Gelman. 

Brian,

As a prologue, I cced Andrew Gelman who is a professor of Poli Sci at Columbia and a very prominent Bayesian Statistician. He too has written on the sparsity of Bayesian economics and I cced him because I thought he too would find this email of interest given his very public advocacy of all methods Bayesian in science (and social sciences!) and having had interactions with Andrew before, I know he may even want to post this on his own blog! (seehttp://andrewgelman.com/  -- It is truly high quality).

The comments section on your post (on the blog posthttp://econlog.econlib.org/archives/2009/11/why_arent_acade.html#91157) are closed which is most natural due to its age.

I thought nonetheless I should write to you about Bayesian Econometricians as an addendum. Obviously some commentatorswere wrongly disposed to think economists do not have the bandwidth to understand Bayes’ rule. It is a much deeper historical (hysteresis) problem. I would not say that Phillips or Granger do not have the bandwidth—far from it. Surprisingly, they are wrong-headedabout their philosophy as though the Pearson-Neymann Fisherphilosophy (repeated experiments, really now? In economics? Guffaw! LoL!) was the most applicable to economics alone even though it is increasingly out of favour in the physical sciences. This is so wrong it is hard to fathom. Meanwhile, most economists I speak with know full well how Bayes’ rule works if they have never actually used it in practice!

In physics, many prominent physicists are active supporters of the Many Worlds Interpretation (i.e., http://en.wikipedia.org/wiki/Many-worlds_interpretation) as opposed to the dominant Copenhagen Interpretation -  Hawkings among others. Edwin Jaynes claimed that the Copenhagen school with its’ god-like Observers was a form of Mind Projection Fallacy. The big difference is Jaynes and Jeffreys(and Hawkings) all believe that probability is in the mind of the beholder. It measures our own lack of information. For Einstein, God does not roll dice. Reality is all deterministic even if the equations are not.

I don’t see why Economics should be different. We do not know how economic agents will act. Does that make them “random”? As though randomness was some physical property assigned to possible states of the world? No not in any way. Randomness is in the mind of the person who does not know all. De Finetti and Ramsey, Savage and Jeffreys (and Richard Jeffrey, see e.g.,http://en.wikipedia.org/wiki/Richard_Jeffrey)  were all onto something very real when it was often quashed by the (then) orthodoxy of Fisherian school with all its triple negatives (“we cannot fail to reject the null hypothesis of X” Tell me who really knows what that means?).Bayes must be accepted if only because Ockham’s razor tells us, the interpretation is far easier in the long run. Meanwhile interpreting probabilities using DBAs (Dutch Book Arguments) a la Ramsey, gives them a very real (risk-neutral) pricing interpretation. Uncertainty then is an inability to find a unique mid. It all makes sense in a self-consistent economics interpretation sort of way. Why rely on ontological arguments for the existence of some other-worldly probability that has physical meaning when it only need be a unique price a la information markets!?!?

What is truly shocking is that  Statistics departments have come fullswing and are full of Bayesians or agnostics (e.g., Donald Rubin and Brad Efron ), who use both Bayesian and (modern) frequentist methods (e.g., Bootstrap, Cross validation, frequentist nonparametrics) whenever they suit the problem to hand. But some of the best Bayesian stats goes on outside statistics departments (e.g.,in Poli Sci, see Andrew Gelman’s website, http://andrewgelman.com/ or Simon Jackman, http://jackman.stanford.edu/blog/ or in Physics and Comp Sci Departments, usually in areas of Machine Learning, e.g., David MacKay, Judea Pearl, et al).  Nonetheless this world outside economics is Bayes’ friendly! (see e.g.,http://videolectures.net/ for a plethora of pro-Bayes’ lectures from CS and other areas, and see http://www.youtube.com/watch?v=8oD6eBkjF9o for the reception that the likes of Google gives authors who advocate Bayes’ rule—author S B McGrayne of The Theory That would Not Die). Bayesian methods now form the basis for much of Machine Learning, probably the most successful version of Artificial Intelligence there is, giving good reason for theirprominence in CS departments.

But Econometrics departments are full of die-hard Frequentists --Phillips and Granger and Gouriereux et al with their shockinglycomplex assumptions, complex mathematics, large sample theory (when was an economics sample ever that large? The only large sample is the one they generate using their computers!), etc -surprisingly wedded to their philosophy.

We can probably count the Bayesian Econometricians on the fingers of one hand—Zellner, Sims, Koop, Lancaster-- Maybe that’s it? (see e.g., http://en.wikipedia.org/wiki/Bayesian_econometrics).  I don’t understand why! Some areas (e.g., I(0), I(1)-stationarity, cointegration theory) do not lend themselves to Bayesian analysis (Sims famously called it a prior on a set of measure 0, to which Phillips replied that the Jeffreys’ prior (for a hypothesis not a model??? I never heard of a Jeffreys’ prior for a hypothesis before this or after this paper!) should in fact have a Dirac mass appropriately placed). In the end, Sims’analysis is more elegant, less formalistic, easier to understand and allows plenty of explosive forecasts in the predictive densities! Some Bayesians have tried to wed the two although it makes for the unnatural priors on concentrated on cones (of rank one matrices!).Much easier to forget it all and use a Minnesota Prior!

And surprisingly, virtually every economist and finance PhD I have ever spoken with talks regularly about their priors and posteriors in reference to models, to visions of reality, etc. All of their principal agent models have some sort of prior or posterior in them even if Bayesian updating is not explicit. It figures well into the way they seereality. Most theoretical economists/finance PhDs will have complicated (DSGE) models for reality which, when it comes down to it, justify a few RHS variables in a simple regression and they do an OLS or GLS if they’re really sophisticated, and are done with it. Theyopt for ease (and linearity) in their PhD theses!  In fact, I think F Black was one of the few theoreticians who didn’t seem to take on Bayes’wholeheartedly (see his paper on Noise-- http://www.e-m-h.org/Blac86.pdf). He speaks of knowledge all the time but forgets Bayes! It is clear that epistemological interpretations are far easier than others.

But the econometricians are all die-hard. Why?  It is econometricians who are the truly egregious sinners here, not wanting to state what they believe because it in the end sounds just plain ridiculous. This state of affairs is enough to make one opt for the religious fervour of Bayesians as one can see in the zeal of Zellner or the stridency of Sims (to alliterate!). I can only imagine it is because the math is harder. As a former math (asst) prof who used to do lots of convergence theory (weak-star in the sense of measures, weal in this or that Sobolev sense, etc), there is an appeal to the mostly impenetrable and completely unintuitive analysis. Quasi-MLE and simulated method of moments. Wow. Amazing what you can do with millions of computer generated observations. Too bad we only have one historical path in reality—the stock market only did as it did once and we can’t seem to generate any other reasonable histories -- As a realist I see little sense in the methods.

For me, a PhD in Applied Math (in Nonlinear PDE) who never used a statistical method before learning (the frequentist) methods in fashion in Wall St, I found Bayes’ theory and philosophy a major relief. No more triple negatives. No more having to explain the craziness of hypothesis testing and why you chose 10% rather than 5% or 1% as a CI (because I got the results I wanted????). I only wish I could be more of a Bayesian in practice!!!

I might even liken the use of Bayesian methods in Economics to the use of Sabrmetrics in Baseball. There will come a time when the revolution happens and the frequentists will just have to stand back and give in, or be swamped.

Should we make it an active plan to promote Bayesian methods in Economics departments? To grow a new generation of Econometricians who are freed from the shackles of overly formalistic large sample theory favoured by frequentists? Should all intro courses in Econometrics be taught out of Koophttp://www.amazon.co.uk/Bayesian-Econometrics-Gary-Koop/dp/0470845678  or Lancasterhttp://www.amazon.co.uk/Introduction-Modern-Bayesian-Econometrics/dp/1405117206/ref=sr_1_4?s=books&ie=UTF8&qid=1373363994&sr=1-4&keywords=bayesian+econometrics?   Should we actively work on the demise of a school of philosophy (frequentist theory or its “bastard child” propensity theory) which are so obviously lacking in any sensible interpretation?

Any comments or answers to my queries would be most welcome.

Best regards,

Nick Firoozye

_____________________________________________

Jaynes on the Mind

Edwin Jaynes: some refs 

The major thesis-Randomness is all in the mind. Epistemological interpretations of probability. Bayesian objectivist and sometimes subjectivist (a la de Finetti and Ramsey). Big believer in Jeffreys and the use of his objectivist priors or maxent methods for fixing (maximally uninformative) priors. 

The book:

http://www.amazon.co.uk/dp/0521592712

Probability Theory: The Logic of Science: Principles and Elementary Applications Vol 1 [Hardcover]  (only volume 1 available)

 

 

The entire book as PDF:

 

http://pjp-75.uio.no/backup/Salvage/Game.Design.eBooks.Pack/Programming/Probability%20Theory%20-%20The%20Logic%20of%20Science.pdf?bcsi_scan_dd0fad490e5fad80=0&bcsi_scan_filename=Probability%20Theory%20-%20The%20Logic%20of%20Science.pdf

 

 

The website:

http://bayes.wustl.edu/  (worth a look, links to all). 

 

 

http://en.wikipedia.org/wiki/Edwin_Thompson_Jaynes Real strident fellow. Think he may have been a Republican? Or maybe it was just the fashion of the times!

 

Jaynes’ attack on Copenhagen Interpretation

http://en.wikipedia.org/wiki/Mind_projection_fallacy

 

 

Refs on Interpretations of Quantum

http://en.wikipedia.org/wiki/Interpretation_of_quantum_mechanics (table pasted below)

 

 

http://en.wikipedia.org/wiki/Minority_interpretations_of_quantum_mechanics

 

Note that many interpretations including the now popular many worlds interpretation do not require a belief in physical randomness and instead are fully deterministic.  

 

nterpretation	Author(s)	Deterministic?	Wavefunction real?	Unique history?	Hidden variables?	Collapsing wavefunctions?	Observer role?	Local?	Counterfactual definiteness?	Universal wavefunction exists?
Ensemble interpretation	Max Born, 1926	Agnostic	No	Yes	Agnostic	No	None	No	No	No
Copenhagen interpretation	Niels Bohr,Werner Heisenberg, 1927	No	No1	Yes	No	Yes2	Causal	No	No	No
de Broglie–Bohm theory	Louis de Broglie, 1927,David Bohm, 1952	Yes	Yes3	Yes4	Yes	No	None	No	Yes	Yes
von Neumann interpretation	John von Neumann, 1932,John Archibald Wheeler,Eugene Wigner	No	Yes	Yes	No	Yes	Causal	No	No	Yes
Quantum logic	Garrett Birkhoff, 1936	Agnostic	Agnostic	Yes5	No	No	Interpretational6	Agnostic	No	No
Many-worlds interpretation	Hugh Everett, 1957	Yes	Yes	No	No	No	None	Yes	No	Yes
Popper's interpretation[46]	Karl Popper, 1957[47]	No	Yes	Yes	Yes	No	None	Yes	Yes13	No
Time-symmetric theories	Yakir Aharonov, 1964	Yes	Yes	Yes	Yes	No	No	Yes	No
Stochastic interpretation	Edward Nelson, 1966	No	No	Yes	No	No	None	No	No	No
Many-minds interpretation	H. Dieter Zeh, 1970	Yes	Yes	No	No	No	Interpretational7	Yes	No	Yes
Consistent histories	Robert B. Griffiths, 1984	Agnostic8	Agnostic8	No	No	No	Interpretational6	Yes	No	No
Objective collapse theories	Ghirardi–Rimini–Weber, 1986, Penrose interpretation, 1989	No	Yes	Yes	No	Yes	None	No	No	No
Transactional interpretation	John G. Cramer, 1986	No	Yes	Yes	No	Yes9	None	No14	Yes	No
Relational interpretation	Carlo Rovelli, 1994	No	No	Agnostic10	No	Yes11	Intrinsic12	Yes	No	No