Spread of Ideas in Social Networks

Social networks have existed for millennia. In schools, fraternities, clubs and associations form various networks within the campus. In job hunting, networking is essential. Ideas spread across various academic circles, while within a school of thought people have some common ideas. Various intelligence agencies study extensively a terrorist organization by understanding their network structure.

Recently, Damon Centola from University of Pennsylvania studied how social networks form and what that means for the ideas that will spread across them. [Centola 2015] It is based on a study by social theorists Peter Blau and Joseph Schwartz in 1984, who argued that a society with eliminated group boundaries enjoys the greatest level of social integration. [Blau & Schwarts 1984] These group boundaries are due to differences in cultures, races, religions, income, levels of education, hobbies, political party etc. Their study implied that a totally mixed society has the greatest level of social integration. However, Centola’s study built on this idea and developed further. He found that a society with completely eliminated boundaries ultimately reduces social integration.

In a diverse society like America, we wish to achieve total social integration to allow the widest spread of complex ideas. However, Centola’s finding indicates that while a total segregation is not desired, a moderate boundary is needed for social integration. Associations that are based on cultures, races, hobbies etc. are actually essential for the development of societies and spread of ideas.

There are many other similar studies in the past. In a celebrated paper authored by Mark Granovetter, the impact of “weak ties” are strong on the diffusion of influence and information. [Gravovetter 1973] The study by Thomas Schelling, a Nobel Laureate in Economics, also studied that the complete tolerance does not mean social stability. [Schelling 1978]

Analytics researchers can study the social network computationally using the Python package networkx.

(Taken from Phys.org)

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Stochastics and Sentiment Analysis in Wall Street

Wall Street is not only a place of facilitating the money flow, but also a playground for scientists.

When I was young, I saw one of my uncles plotting prices for stocks to perform technical analysis. When I was in college, my friends often talked about investing in a few financial futures and options. When I was doing my graduate degree in physics, we studied John Hull’s famous textbook [Hull 2011] on quantitative finance to learn about financial modeling. A few of my classmates went to Wall Street to become quantitative analysts or financial software developers. There are ups and downs in the financial markets. But as long as we are in a capitalist society, finance is a subject we never ignore. However, scientists have not come up with a consensus about the nature of a financial market.

Agent-Based Models

Economists believe that individuals in a market are rational being who always aim at maximizing their profits. They often apply agent-based models, which employs complex system theories or game theory.

Random Processes and Statistical Physics

However, a lot of mathematicians in Wall Street (including quantitative analysts and econophysicists) see the stock prices as undergoing Brownian motion. [Hull 2011, Baaquie 2007] They employ tools in statistical physics and stochastic processes to study the pricings of various financial derivatives. Therefore, the random-process and econophysical approaches have nothing much about stock price prediction (despite the fact that they do need a “return rate” in their model.) Random processes are unpredictable.

However, some sort of predictions carry great values. For example, when there is overhypes or bubbles in the market, we want to know when it will burst. There are models that predict defaults and bubble burst in a market using the log-periodic power law (LPPL). [Wosnitza, Denz 2013] In addition, there has been research showing the leverage effect in stock markets in developed countries such as Germany (c.f. fluctuation-dissipation theorem in statistical physics), and anti-leverage effect in China (Shanghai and Shenzhen). [Qiu, Zhen, Ren, Trimper 2006]

Reconciling Intelligence and Randomness

There are some values to both views. It is hard to believe that stock prices are completely random, as the economic environment and the public opinions must affect the stock prices. People can neither be completely rational nor completely random.

There has been some study in reconciling game theory and random processes, in an attempt to bring economists and mathematicians together. In this theoretical framework, financial systems still sought to attain the maximum entropy (randomness), but the “particles” in the system behaves intelligently. [Venkatasubramanian, Luo, Sethuraman 2015] (See my another blog entry: MathAnalytics (1) – Beautiful Mind, Physical Nature and Economic Inequality) We are not sure how successful this attempt will be at this point.

Sentiment Analysis

As people are talking about big data in recent years, there have been attempts to apply machine learning algorithms in finance. However, scientists tend not to price using machine learning algorithms because these algorithms mostly perform classification. However, there are attempts, with natural language processing (NLP) techniques, to predict the stock prices by detecting the public emotions (or sentiments) in social media such as Twitter. [Bollen, Mao, Zeng 2010] It has been found that measuring the public mood in a few dimensions (including Calm, Alert, Sure, Vital, Kind, and Happy) allows scientists to accurately predict the trend of Dow Jones Industrial Average (DJIA). However, some hackers take advantage on the sentiment analysis on Twitter. In 2013, there was a rumor on Twitter saying the White House being bombed, The computers responded instantly and automatically by performing trading, causing the stock market to fall immediately. But the market restored quickly after it was discovered that the news was fake. (Fig. 1)

Fig. 1: DJIA fell because of a rumor of the White House being bombed, but recovered when discovered the news was fake (taken from http://www.rt.com/news/syrian-electronic-army-ap-twitter-349/)

P.S.: While I was writing this, I saw an interesting statement in the paper about leverage effect. [Qiu, Zhen, Ren, Trimper 2006] The authors said that:

Why do the German and Chinese markets exhibit different return-volatility correlations? Germany is a developed country. To some extent, people show risk aversion, and therefore, may be nervous in trading as the stock price is falling. This induces a higher volatility. When the price is rising, people feel safe and are inactive in trading. Thus, the stock price tends to be stable. This should be the social origin of the leverage effect. However, China just experiences the first stage of capitalism, and people are somewhat excessive speculative in the financial markets. Therefore, people rush for trading as the stock price increases. When the price drops, people stay inactive in trading and wait for rising up of the stock price. That explains the antileverage effect.

Does this paragraph written in 2006 give a hint of what happened in China in 2015 now? (Fig. 2)

Fig. 2: The fall of Chinese stock market in 2015 (taken from http://www.economicpolicyjournal.com/2015/06/breaking-biggest-chinese-stock-market.html)

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Choices of Tools

When dealing with data analytics, what kind of things do we usually spend most of our time on?

I would say data cleaning and modeling.

Therefore, it is not merely software development. While we sometimes spend a lot of time in software architecture (which is important), before doing that, we have to explore what we want. Very often, data come in various formats, or we need to manually clean them. And very often we do not know which algorithms to use. We need to explore different ways to perform the experiments before determining what to include in the software project.

That’s why interactive programming comes into place for analytics project. R and MATLAB are these examples. However, they provide poor support for modularizing the codes. Python is a good tool that supports both modularization and interactive programming, but it takes an environment to run Python, which is very often a pain. Provided that a lot of good libraries are written in Java, having the need to perform both software development and data analytics, Scala, a JVM language that supports interactive programming, will be the next generation of programming language.

IMG_20150107_201432

Learning by Zooming Out

Deep learning, a collection of related neural network algorithms, has been proved successful in certain types of machine learning tasks in computer vision, speech recognition, data cleaning, and natural language processing (NLP). [Mikolov et. al. 2013] However, it was unclear how deep learning can be so successful. It looks like a black box with messy inputs and excellent outputs. So why is it so successful?

A friend of mine showed me this article in the preprint (arXiv:1410.3831) [Mehta & Schwab 2014] last year, which mathematically shows the equivalence of deep learning and renormalization group (RG). RG is a concept in theoretical physics that has been widely applied in different problems, including critical phenomena, self-organized criticality, particle physics, polymer physics, and strongly correlated electronic systems. And now, Mehta and Schwab showed that an explanation to the performance of deep learning is available through RG.

[Taken from http://www.inspiredeconomies.com/intelligibleecosystems/images/fractals/GasketMag.gif]

So what is RG? Before RG, Leo Kadanoff, a physics professor in University of Chicago, proposed an idea of coarse-graining in studying many-body problems in 1966. [Kadanoff 1966] In 1972, Kenneth Wilson and Michael Fisher succeeded in applying ɛ-expansion in perturbative RG to explain the critical exponents in Landau-Ginzburg-Wilson (LGW) Hamiltonian. [Wilson & Fisher 1972] This work has been the standard material of graduate physics courses. In 1974, Kenneth Wilson applied RG to explain the Kondo problem, which led to his Nobel Prize in Physics in 1982. [Wilson 1983]

RG assumes a system of scale invariance, which means the system are similar in whatever scale you are seeing. One example is the chaotic system as in Fig. 1. The system looks the same when you zoom in. We call this scale-invariant system self-similar. And physical systems closed to phase transition are self-similar. And if it is self-similar, Kadanoff’s idea of coarse-graining is then applicable, as in Fig. 2. Four spins can be viewed as one spin that “summarizes” the four spins in that block without changing the description of the physical system. This is somewhat like we “zoom out” the picture on Photoshop or Web Browser.

[Taken from [Singh 2014]]

So what’s the point of zooming out? Physicists care about the Helmholtz free energies of physical systems, which are similar to cost functions to the computer scientists and machine learning specialists. Both are to be minimized. However, whatever scale we are viewing at, the energy of the system should be scale-invariant. Therefore, as we zoom out, the system “changes” yet “looks the same” due to self-similarity, but the energy stays the same. The form of the model is unchanged, but the parameters change as the scale changes.

This is important, because this process tells us which parameters are relevant, and which others are irrelevant. Why? Think of it this way: we have an awesome computer to simulate a glass of water that contains 1023 water molecules. To describe the systems, you have all parameters, including the position of molecules, strength of Van der Waals force, orbital angular momentum of each atom, strength of the covalent bonds, velocities of the molecules… You might have 1025 parameters. However, this awesome computer cannot handle such a system with so many parameters. Then you try to coarse-grain the system, and you discard some parameters in each step of coarse-graining. After numerous steps, it turns out that the temperature and the pressure are the only relevant parameters.

RG helps you identify the relevant parameters.

And it is exactly what happened in deep learning. In each convolutional cycle, features that are not important are gradually discarded, and those that are important are kept and enhanced. Indeed, in computer vision and NLP, the data are so noisy that there are a lot of unnecessary information. Deep learning gradually discards these information. As Mehta and Schwab stated, [Mehta & Schwab 2014]

Our results suggests that deep learning algorithms may be employing a generalized RG-like scheme to learn relevant features from data.

So what is the point of understanding this? Unlike other machine algorithms, we did not know how it works, which sometimes makes model building very difficult because we have no idea how to adjust parameters. I believe understanding its equivalence to RG helps guide us to build a model that works.

Charles Martin also wrote a blog entry with more demonstration about the equivalence of deep learning and RG. [Martin 2015]

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