Re-read Pearl, and read the Heckman-Pinto critique.I'm sure I'll blog on some of the above in due course. Meanwhile I want to add something to the list: Pearl's response to Heckman-Pinto. (I just learned of it.)
Re-read White et al. on settable systems and testing conditional independence.
Read Angrist-Pischke.
Read Wolpin, and Rust's review.
Read Dawid 2007 and Dawid 2014.
Get and read Imbens-Rubin (available early 2015).
If you were told that in a particular match, Player A won more points and more games and had a higher first-serve percentage, fewer unforced errors and a higher winning percentage at the net, you would deduce that Player A was the winner. But leaping to that conclusion would be a mistake. ... In tennis, it is not the events that constitute a match, but the timing of those events. In team sports like baseball, basketball and football, and even in boxing, the competitor who scores first or last may have little bearing on the outcome. In tennis, the player who scores last is always the winner.Tricky tennis scoring makes for tricky match summarization, whether graphically or otherwise. Not that people haven't tried, with all sorts of devices in use. See, for example, another good 2014 New York Times piece, "How to Keep Score: However You Like," and the fascinating GameSetMap.com, "A blog devoted to maps about tennis," emphasizing spatial aspects but going farther on occasion.
Quantum computing uses an entirely different approach than classical computing. A useful analogy is to think of a landscape with mountains and valleys. Solving optimization problems can be thought of as trying to find the lowest point on this landscape. Every possible solution is mapped to coordinates on the landscape, and the altitude of the landscape is the “energy’” or “cost” of the solution at that point. The aim is to find the lowest point on the map and read the coordinates, as this gives the lowest energy, or optimal solution to the problem. Classical computers running classical algorithms can only "walk over this landscape". Quantum computers can tunnel through the landscape making it faster to find the lowest point.Remember the old days of "math co-processors"? Soon you may have a "quantum co-processor" for those really tough optimization problems! And you thought you were cool if you had a GPU or two.
A special purpose "quantum computer" manufactured by the Canadian company D-Wave has led to intense excitement in the mainstream media (including a Time magazine cover dubbing it "the infinity machine") and the computer industry, and a lively debate in the academic community. Scientifically it leads to the interesting question of whether it is possible to obtain quantum effects on a large scale with qubits that are not individually well protected from decoherence.Wow. I'm there.
We propose a simple and natural classical model for the D-Wave machine - replacing their superconducting qubits with classical magnets, coupled with nearest neighbor interactions whose strength is taken from D-Wave's specifications. The behavior of this classical model agrees remarkably well with posted experimental data about the input-output behavior of the D-Wave machine.
Further investigation of our classical model shows that despite its simplicity, it exhibits novel algorithmic properties. Its behavior is fundamentally different from that of its close cousin, classical heuristic simulated annealing. In particular, the major motivation behind the D-Wave machine was the hope that it would tunnel through local minima in the energy landscape, minima that simulated annealing got stuck in. The reproduction of D-Wave's behavior by our classical model demonstrates that tunneling on a large scale may be a more subtle phenomenon than was previously understood...
One of the remarkable discoveries of the last quarter century is that quantum many-body systems violate the extended Church-Turing thesis and exhibit exponential complexity -- describing the state of such a system of even a few hundred particles would require a classical memory larger than the size of the Universe. This means that a test of quantum mechanics in the limit of high complexity would require scientists to experimentally study systems that are inherently exponentially more powerful than human thought itself!
A little reflection shows that the standard scientific method of "predict and verify" is no longer viable in this regime, since a calculation of the theory's prediction is rendered computationally infeasible. Does this mean that it is impossible to do science in this regime? A remarkable connection with the theory of interactive proof systems (the crown jewel of computational complexity theory) suggests a potential way around this impasse: interactive experiments. Rather than carry out a single experiment, the experimentalist performs a sequence of experiments, and rather than predicting the outcome of each experiment, the experimentalist checks a posteriori that the outcomes of the experiments are consistent with each other and the theory to be tested. Whether this approach will formally work is intimately related to the power of a certain kind of interactive proof system; a question that is currently wide open. Two natural variants of this question have been recently answered in the affirmative, and have resulted in a breakthrough in the closely related area of testing untrusted quantum devices.Wow. Now my head is really spinning. I'm there too, for sure.
I am happy to help with your evaluation of Professor zzz. This email will serve as my letter. [email here]...Countless times, from Chair/Dean xxx to me:
Thanks very much for your thoughtful evaluation. Can you please put it on your university letterhead and re-send?Fantasy response from me to Chair/Dean xxx:
Sure, no problem at all. My time is completely worthless, so I'm happy to oblige, despite the fact that email conveys precisely the same information and is every bit as legally binding (whatever that even means in this context) as a "signed" "letter" on "letterhead." So now I’ll copy my email, try to find some dusty old Word doc letterhead on my hard drive, paste the email into the Word doc, try to beat it into submission depending on how poor the formatting / font / color / blocking looks when first pasted, print from Word to pdf, attach the pdf to a new email, and re-send it to you. How 1990’s.Actually last week I did send something approximating the fantasy email to a dean at a leading institution. I suspect that he didn't find it amusing. (I never heard back.) But as I also said at the end of that email,
"Please don’t be annoyed. I...know that these sorts of 'requirements' have nothing to do with you per se. Instead I’m just trying to push us both forward in our joint battle with red tape."
Consider, for example, the following folk theorem: "Under asymmetric loss, the optimal prediction is conditionally biased." The folk theorem is false. But how can that be?What's true is this: The conditional mean is the L-optimal forecast if and only if the loss function L is in the Bregman family, given by
If \(y_{t}\) is a conditionally Gaussian process and \( L(e_{t+h} )\) is any loss function defined on the \(h\)-step-ahead prediction error \(e_{t+h |t}\), then the \(L\)-optimal predictor is of the form \begin{equation} y_{t+h | t} = \mu _{t+h,t} + \alpha _{t}, \end{equation}where \( \mu _{t+h,t} = E(y_{t+h} | \Omega_t) \), \( \Omega_t = y_t, y_{t-1}, ...\), and \(\alpha _{t}\) depends only on the loss function \(L\) and the conditional prediction-error variance \( var(e _{t+h} | \Omega _{t} )\).
You will not hear from the organizers unless they decide to use your paper.It started with one leading group's calls, which go so even farther:
You will not hear from the organizers unless they decide to use your paper. They are not journal editors or program committee chairmen for a society.Now it's spreading.
Nassim Taleb is a financial markets trader (and Wharton graduate) turned pop author. His book, The Black Swan, deals with many of the issues raised in this chapter. "Black swans"' are seemingly impossible or very low-probability events -- after all, swans are supposed to be white -- that occur with annoying regularity in reality. Read his book. Where does your reaction fall on the A-B spectrum below?
A. Taleb offers crucial lessons for econometricians, heightening awareness in ways otherwise difficult to achieve. After reading Taleb, it's hard to stop worrying about non-normality, model misspecification, and so on.
B. Taleb belabors the obvious for hundreds of pages, arrogantly "informing"' us that non-normality is prevalent, that all models are misspecified, and so on. Moreover, it takes a model to beat a model, and Taleb offers nothing new.The book is worth reading, regardless of where your reaction falls on the A-B spectrum.
A last thought. Economic Fluctuations [an NBER program] merged with Growth [another NBER program] in the mid 1990s. At the time there was a great confluence of method as well as interest. Growth theorists were studying growth with Bellman equations, dynamic general equilibrium models of innovation and transmission of ideas, thinking about where productivity shocks came from. Macroeconomists were using Bellman equations and studying dynamic general equilibrium models with stochastic technology, along with various frictions and other propagation mechanisms.
That confluence has now diverged. ... ...when Daron Acemoglu, who seems to know everything about everything, has to preface his comments on macro papers with repeated disclaimers of lack of expertise, it's clear that the two fields [fluctuations and growth] really have gone their separate ways. Perhaps it's time to merge fluctuations with finance, where we seem to be talking about the same issues and using the same methods, and to merge growth with institutions and political or social economics.I agree. Fluctuations and finance belong together. (I'm talking about asset pricing broadly defined, not corporate finance.) Yes, the methods are basically the same, and moreover, the substance is inextricably linked. Aspects of fluctuations are effectively the fundamentals priced in financial markets, and conversely, financial markets can most definitely impact fluctuations. (Remember that little recession a few years back?)
[Material in square brackets and bold added by me.]
| ECB Eurotower |
"Piketty's book truly shines on the data side. ... Its tables and figures...provide a rich and jaw-dropping image, like a new high-resolution photo of a previously-unseen galaxy. I'm grateful to Piketty for sending it our way, for heightening awareness, and for raising important questions."Measurement endorsements don't come much stronger.
People keep referring to economists who have favorable views of Piketty's book. Leaving aside Krugman, I would be interested in knowing the name of any economist who asserts that Piketty's reasoning ... is other than gibberish.