Sunday, January 13, 2019

Machine Learning & Econometrics

What is Machine Learning (ML), and how does it differ from Statistics (and hence, implicitly, from Econometrics)?

Those are big questions, but I think that they're ones that econometricians should be thinking about. And if I were starting out in Econometrics today, I'd take a long, hard look at what's going on in ML.

Here's a very rough answer - it comes from a post by Larry Wasserman on his (now defunct) blog, Normal Deviate:
"The short answer is: None. They are both concerned with the same question: how do we learn from data?
But a more nuanced view reveals that there are differences due to historical and sociological reasons.......... 
If I had to summarize the main difference between the two fields I would say: 
Statistics emphasizes formal statistical inference (confidence intervals, hypothesis tests, optimal estimators) in low dimensional problems. 
Machine Learning emphasizes high dimensional prediction problems. 
But this is a gross over-simplification. Perhaps it is better to list some topics that receive more attention from one field rather than the other. For example: 
Statistics: survival analysis, spatial analysis, multiple testing, minimax theory, deconvolution, semiparametric inference, bootstrapping, time series.
Machine Learning: online learning, semisupervised learning, manifold learning, active learning, boosting. 
But the differences become blurrier all the time........ 
There are also differences in terminology. Here are some examples:
Statistics       Machine Learning
———————————–
Estimation        Learning
Classifier          Hypothesis
Data point         Example/Instance
Regression        Supervised Learning
Classification    Supervised Learning
Covariate          Feature
Response          Label 
Overall, the the two fields are blending together more and more and I think this is a good thing."
As I said, this is only a rough answer - and it's by no means a comprehensive one.

For an econometrician's perspective on all of this you can't do better that to take a look at Frank Dielbold's blog, No Hesitations. If you follow up on his posts with the label "Machine Learning" - and I suggest that you do - then you'll find 36 of them (at the time of writing).

If (legitimately) free books are your thing, then you'll find some great suggestions for reading more about the Machine Learning / Data Science field(s) on the KDnuggets website - specifically, here in 2017 and here in 2018.

Finally, I was pleased that the recent ASSA Meetings (ASSA2019) included an important contribution by Susan Athey (Stanford), titled "The Impact of Machine Learning on Econometrics and Economics". The title page for Susan's presentation contains three important links to other papers and a webcast.

Have fun!

© 2018, David E. Giles

Friday, January 11, 2019

Shout-out for Mischa Fisher

One of my former grad. students, Mischa Fisher, is currently Chief Economist and Advisor to the Governor of the State of Illinois. In this role he has oversight of a number of State agencies dealing with economics and data science.

This week, he had a really nice post on the Datascience.com blog. It's titled "10 Data Science Pitfalls to Avoid".

Mischa is very knowledgeable, and he writes extremely well. I strongly recommend that you take a look at his piece.

© 2018, David E. Giles

Monday, January 7, 2019

Bradley Efron and the Bootstrap

Econometricians make extensive use of various forms of "The Bootstrap", thanks to Bradley (Brad) Efron's pioneering work.

I've posted about the history of the bootstrap previously - e.g., here, and here.

You probably know by now that Brad was awarded The International Prize in Statistics last November - this was only the second time that this prize has been awarded. It's difficult to think of a more deserving recipient.


If you want to read an excellent account of Brad's work, and how the bootstrap came to be, I recommend the 2003 piece by Susan Holmes, Carl Morris, and Rob Tibshirani.

There are some fascinating snippets in this conversation/interview, including:
Efron: "One of the reasons I came to Stanford was because of its humor magazine. I wrote a humor column at Caltech, and I always wanted to write for a humor magazine. Stanford had a great humor magazine, The Chaparral. The first few months I was there, the editor literally went crazy and had to be hospitalized, and so I became editor. For one issue we did a parody of Playboy and it went a little too far. I was expelled from school, ..... I went away for 6 months and then I came back. That was by far the most famous I’ve ever been." 
 Referring to his seminal paper (Efron, 1979):
Tibshirani: "It was sent to the Annals. What kind of reception did it get?" 
Efron: "Rupert Miller was the editor of the Annals at the time. I submitted what was the Rietz lecture, and it got turned down. The associate editor, who will remain nameless, said it that didn’t have any theorems in it. So, I put some theorems in at the end and put a lot of pressure on Rupert, and he finally published it."
I guess there's still hope for the rest of us!

References

Efron, B., 1979. Bootstrap methods: Another look at the jackknife. Annals of Statistics, 7, 1-26.

Holmes, S., C. Morris, & R. Tibshirani, 2003. Bradley Efron: A conversation with good friends. Statistical Science, 18, 268-281.

© 2018, David E. Giles

Tuesday, January 1, 2019

New Year Reading Suggestions for 2019

With a new year upon us, it's time to keep up with new developments -
  • Basu, D., 2018. Can we determine the direction of omitted variable bias of OLS estimators? Working Paper 2018-16, Department of Economics, University of Massachusetts, Amherst.
  • Jiang, B., Y. Lu, & J. Y. Park, 2018. Testing for stationarity at high frequency. Working Paper 2018-9, Department of Economics, University of Sydney. 
  • Psaradakis, Z. & M. Vavra, 2018. Normality tests for dependent data: Large-sample and bootstrap approaches. Communications in Statistics - Simulation and Computation, online.
  • Spanos, A., 2018. Near-collinearity in linear regression revisited: The numerical vs. the statistical perspective. Communications in Statistics - Theory and Methods, online.
  • Thorsrud, L. A., 2018. Words are the new numbers: A newsy coincident index of the business cycle. Journal of Business Economics and Statistics, online. (Working Paper version.)
  • Zhang, J., 2018. The mean relative entropy: An invariant measure of estimation error. American Statistician, online.
© 2018, David E. Giles