Embedding algorithms, especially word-embedding algorithms, have been one of the recurrent themes of this blog. Word2Vec has been mentioned in a few entries (see this); LDA2Vec has been covered (see this); the mathematical principle of GloVe has been elaborated (see this); I haven’t even covered Facebook’s fasttext; and I have not explained the widely used t-SNE and Kohonen’s map (self-organizing map, SOM).

I have also described the algorithm of Sammon Embedding, (see this) which attempts to capture the likeliness of pairwise Euclidean distances, and I implemented it using Theano. This blog entry is about its implementation in Tensorflow as a demonstration.

Let’s recall the formalism of Sammon Embedding, as outlined in the previous entry:

Assume there are high dimensional data described by $d$-dimensional vectors, $X_i$ where $i=1, 2, \ldots, N$. And they will be mapped into vectors $Y_i$, with dimensions 2 or 3. Denote the distances to be $d_{ij}^{*} = \sqrt{| X_i - X_j|^2}$ and $d_{ij} = \sqrt{| Y_i - Y_j|^2}$. In this problem, $Y_i$ are the variables to be learned. The cost function to minimize is $E = \frac{1}{c} \sum_{i,

where $c = \sum_{i.

Unlike in previous entry and original paper, I am going to optimize it using first-order gradient optimizer. If you are not familiar with Tensorflow, take a look at some online articles, for example, “Tensorflow demystified.” This demonstration can be found in this Jupyter Notebook in Github.

First of all, import all the libraries required:

import numpy as np
import matplotlib.pyplot as plt
import tensorflow as tf


Like previously, we want to use the points clustered around at the four nodes of a tetrahedron as an illustration, which is expected to give equidistant clusters. We sample points around them, as shown:

tetrahedron_points = [np.array([0., 0., 0.]), np.array([1., 0., 0.]), np.array([np.cos(np.pi/3), np.sin(np.pi/3), 0.]), np.array([0.5, 0.5/np.sqrt(3), np.sqrt(2./3.)])]

sampled_points = np.concatenate([np.random.multivariate_normal(point, np.eye(3)*0.0001, 10) for point in tetrahedron_points])

init_points = np.concatenate([np.random.multivariate_normal(point[:2], np.eye(2)*0.0001, 10) for point in tetrahedron_points])


Retrieve the number of points, N, and the resulting dimension, d:

N = sampled_points.shape
d = sampled_points.shape


One of the most challenging technical difficulties is to calculate the pairwise distance. Inspired by this StackOverflow thread and Travis Hoppe’s entry on Thomson’s problem, we know it can be computed. Assuming Einstein’s convention of summation over repeated indices, given vectors $a_{ik}$, the distance matrix is: $D_{ij} = (a_{ik}-a_{jk}) (a_{ik} - a_{jk})^T = a_{ik} a_{ik} + a_{jk} a_{jk} - 2 a_{ik} a_{jk}$,

where the first and last terms are simply the norms of the vectors. After computing the matrix, we will flatten it to vectors, for technical reasons omitted to avoid gradient overflow:

X = tf.placeholder('float')
Xshape = tf.shape(X)

sqX = tf.reduce_sum(X*X, 1)
sqX = tf.reshape(sqX, [-1, 1])
sqDX = sqX - 2*tf.matmul(X, tf.transpose(X)) + tf.transpose(sqX)
sqDXarray = tf.stack([sqDX[i, j] for i in range(N) for j in range(i+1, N)])
DXarray = tf.sqrt(sqDXarray)

Y = tf.Variable(init_points, dtype='float')
sqY = tf.reduce_sum(Y*Y, 1)
sqY = tf.reshape(sqY, [-1, 1])
sqDY = sqY - 2*tf.matmul(Y, tf.transpose(Y)) + tf.transpose(sqY)
sqDYarray = tf.stack([sqDY[i, j] for i in range(N) for j in range(i+1, N)])
DYarray = tf.sqrt(sqDYarray)


And DXarray and DYarray are the vectorized pairwise distances. Then we defined the cost function according to the definition:

Z = tf.reduce_sum(DXarray)*0.5
numerator = tf.reduce_sum(tf.divide(tf.square(DXarray-DYarray), DXarray))*0.5
cost = tf.divide(numerator, Z)


update_rule = tf.assign(Y, Y-0.01*grad_cost/lapl_cost)
init = tf.global_variables_initializer()


The last line initializes all variables in the Tensorflow session when it is run. Then start a Tensorflow session, and initialize all variables globally:

sess = tf.Session()
sess.run(init)


Then run the algorithm:

nbsteps = 1000
c = sess.run(cost, feed_dict={X: sampled_points})
print "epoch: ", -1, " cost = ", c
for i in range(nbsteps):
sess.run(train, feed_dict={X: sampled_points})
c = sess.run(cost, feed_dict={X: sampled_points})
print "epoch: ", i, " cost =


Then extract the points and close the Tensorflow session:

calculated_Y = sess.run(Y, feed_dict={X: sampled_points})
sess.close()


Plot it using matplotlib:

embed1, embed2 = calculated_Y.transpose()
plt.plot(embed1, embed2, 'ro')


This gives, as expected, This code for Sammon Embedding has been incorporated into the Python package mogu, which is a collection of numerical routines. You can install it, and call:

from mogu.embed import sammon_embedding
calculated_Y = sammon_embedding(sampled_points, init_points)


Recently I have been drawn to generative models, such as LDA (latent Dirichlet allocation) and other topic models. In deep learning, there are a few examples, such as FVBN (fully visible belief networks), VAE (variational autoencoder), RBM (restricted Boltzmann machine) etc. Recently I have been reading about GAN (generative adversarial networks), first published by Ian Goodfellow and his colleagues and collaborators. Goodfellow published his talk in NIPS 2016 on arXiv recently.

Yesterday I attended an event at George Mason University organized by Data Science DC Meetup Group. Jennifer Sleeman talked about GAN. It was a very good talk.

In GAN, there are two important functions, namely, the discriminator (D), and the generator (G). As a generative model, the distribution of training data, all labeled positive, can be thought of the distribution that the generator was trained to produce. The discriminator discriminates the data with positive labels and those with negative labels. Then the generator tries to generate data, probably from noises, which should be negative, to fake the discriminator to see it as positive. This process repeats iteratively, and eventually the generator is trained to produce data that are close to the distribution of the training data, and the discriminator will be confused to classify the generated data as positive with probability $\frac{1}{2}$. The intuition of this competitive game is from minimax game in game theory. The formal algorithm is described in the original paper as follow: The original paper discussed about that the distribution of final generated data identical to that of the training data being the optimal for the model, and argued using the Jensen-Shannon (JS) divergence. Ferenc Huszár discussed in his blog about the relations between maximum likelihood, Kullback-Leibler (KL) divergence, and Jensen-Shannon (JS) divergence.

I have asked the speaker a few questions about the concepts of GAN as well.

GAN is not yet a very sophisticated framework, but it already found a few industrial use. Some of its descendants include LapGAN (Laplacian GAN), and DCGAN (deep convolutional GAN). Applications include voice generation, image super-resolution, pix2pix (image-to-image translation), text-to-image synthesis, iGAN (interactive GAN) etc.

Adversarial training is the coolest thing since sliced bread.” – Yann LeCun A while ago, Mehta and Schwab drew a connection between Restricted Boltzmann Machine (RBM), a type of deep learning algorithm, and renormalization group (RG), a theoretical tool in physics applied on critical phenomena. [Mehta & Schwab, 2014; see previous entry] Can RG be able to relate to other deep leaning algorithms?

Schwab wrote a paper on a new machine learning algorithm that directly exploit a type of RG in physics: the density matrix renormalization group (DMRG). DMRG is used in condensed matter physics for low-dimensional (d=1 or 2) lattice systems. DMRG was invented by Steve White, using diagonalization of reduced density matrices on each site. [White 1992] However, now it was performed using singular value decomposition for each successive pair of lattice sites.

DMRG is related to quantum entanglement, which is a two-site quantum system, and the entanglement can be characterized by any of its reduced density matrix. However, DMRG deals with reduced density matrix of all sites. Traditionally, this kind of many body systems can be represented by the kets: $|\Psi \rangle = \sum_{\sigma_1 \ldots \sigma_L} c^{\sigma_1} \ldots c^{\sigma_L} |\sigma_1 \ldots \sigma_L \rangle$.

These c‘s are c-numbers. To describe the entanglement of these states but to remain numerically convenient, it is desirable to convert these c-numbers into matrices: [Schollwöck 2013] $c^{\sigma_1} \ldots c^{\sigma_L} \rightarrow M^{\sigma_1} \ldots M^{\sigma_L}$.

And these are tensor networks. DMRG aims at finding a good description of the states with these tensor networks. These tensor networks have nice graphical representation, as in the appendix of the paper by Stoudenmire and Schwab. The training is also described in their paper elegantly using these tensor network diagrams. Their new algorithm proves to be a good new machine learning algorithm, probably fit for small data but complicated features. This is a direct application of real-space RG in machine learning algorithm. Stoudenmire wrote in Quora about the value of this work:

“In our work… we reached state-of-the-art accuracy for the MNIST dataset without needing extra techniques such as convolutional layers. One exciting aspect of these proposals is that their cost scales at most linearly in the number of training examples, versus quadratically for most kernel methods. Representing parameters by a tensor network gives them a structure that can be analyzed to better understand the model and what it has learned. Also tensor network optimization methods are adaptive, automatically selecting the minimum number of parameters necessary for the optimal solution within a certain tensor network class.” – Miles Stoudenmire, in Quora

There are some extension algorithms from DMRG, such as multiscale entanglement renormalization ansatz (MERA), developed by Vidal and his colleagues. [Vidal 2008] Steve R. White (adapted from his faculty homepage) Tensor Diagram of the Training of this New Algorithm. (Take from arXiv:1605.05775)

Word embedding has been a frequent theme of this blog. But the original embedding has been algorithms that perform a non-linear mapping of higher dimensional data to the lower one. This entry I will talk about one of the most oldest and widely used one: Sammon Embedding, published in 1969. This is an embedding algorithm that preserves the distances between all points. How is it achieved?

Assume there are high dimensional data described by $d$-dimensional vectors, $X_i$ where $i=1, 2, \ldots, N$. And they will be mapped into vectors $Y_i$, with dimensions 2 or 3. Denote the distances to be $d_{ij}^{*} = \sqrt{| X_i - X_j|^2}$ and $d_{ij} = \sqrt{| Y_i - Y_j|^2}$. In this problem, $Y_i$ are the variables to be learned. The cost function to minimize is $E = \frac{1}{c} \sum_{i,

where $c = \sum_{i. To minimize this, use Newton's method by $Y_{pq} (m+1) = Y_{pq} (m) - \alpha \Delta_{pq} (m)$,

where $\Delta_{pq} (m) = \frac{\partial E(m)}{\partial Y_{pq}(m)} / \left|\frac{\partial^2 E(m)}{\partial Y_{pq} (m)^2} \right|$, and $\alpha$ is the learning rate.

To implement it, use Theano package of Python to define the cost function for the sake of optimization, and then implement the learning with numpy. Define the cost function with the outline above:

import theano
import theano.tensor as T

# define variables
mf = T.dscalar('mf')         # magic factor / learning rate

# coordinate variables
Xmatrix = T.dmatrix('Xmatrix')
Ymatrix = T.dmatrix('Ymatrix')

# number of points and dimensions (user specify them)
N, d = Xmatrix.shape
_, td = Ymatrix.shape

# grid indices
n_grid = T.mgrid[0:N, 0:N]
ni = n_grid.flatten()
nj = n_grid.flatten()

# cost function
c_terms, _ = theano.scan(lambda i, j: T.switch(T.lt(i, j),
T.sqrt(T.sum(T.sqr(Xmatrix[i]-Xmatrix[j]))),
0),
sequences=[ni, nj])
c = T.sum(c_terms)

s_term, _ = theano.scan(lambda i, j: T.switch(T.lt(i, j),
T.sqr(T.sqrt(T.sum(T.sqr(Xmatrix[i]-Xmatrix[j])))-T.sqrt(T.sum(T.sqr(Ymatrix[i]-Ymatrix[j]))))/T.sqrt(T.sum(T.sqr(Xmatrix[i]-Xmatrix[j]))),
0),
sequences=[ni, nj])
s = T.sum(s_term)

E = s / c

# function compilation and optimization
Efcn = theano.function([Xmatrix, Ymatrix], E)


And implement the update algorithm with the following function:

import numpy

# training
def sammon_embedding(Xmat, initYmat, alpha=0.3, tol=1e-8, maxsteps=500, return_updates=False):
N, d = Xmat.shape
NY, td = initYmat.shape
if N != NY:
raise ValueError('Number of vectors in Ymat ('+str(NY)+') is not the same as Xmat ('+str(N)+')!')

# iteration
Efcn_X = lambda Ymat: Efcn(Xmat, Ymat)
step = 0
oldYmat = initYmat
oldE = Efcn_X(initYmat)
update_info = {'Ymat': [initYmat], 'cost': [oldE]}
converged = False
while (not converged) and step<=maxsteps:
newE = Efcn_X(newYmat)
if np.all(np.abs(newE-oldE)<tol):
converged = True
oldYmat = newYmat
oldE = newE
step += 1
print 'Step ', step, '\tCost = ', oldE
update_info['Ymat'].append(oldYmat)
update_info['cost'].append(oldE)

# return results
update_info['num_steps'] = step
return oldYmat, update_info
else:
return oldYmat


The above code is stored in the file sammon.py. We can test the algorithm with an example. Remember tetrahedron, a three-dimensional object with four points equidistant from one another. We expect the embedding will reflect this by sampling points around these four points. With the code tetrahedron.py, we implemented it this way:

import argparse

import numpy as np
import matplotlib.pyplot as plt

import sammon as sn

argparser = argparse.ArgumentParser('Embedding points around tetrahedron.')
default='embedded_tetrahedron.png',
help='file name of the output plot')

args = argparser.parse_args()

tetrahedron_points = [np.array([0., 0., 0.]),
np.array([1., 0., 0.]),
np.array([np.cos(np.pi/3), np.sin(np.pi/3), 0.]),
np.array([0.5, 0.5/np.sqrt(3), np.sqrt(2./3.)])]

sampled_points = np.concatenate([np.random.multivariate_normal(point, np.eye(3)*0.0001, 10)
for point in tetrahedron_points])

init_points = np.concatenate([np.random.multivariate_normal(point[:2], np.eye(2)*0.0001, 10)
for point in tetrahedron_points])

embed_points = sn.sammon_embedding(sampled_points, init_points, tol=1e-4)

X, Y = embed_points.transpose()
plt.plot(X, Y, 'x')
plt.savefig(args.output_figurename)


It outputs a graph: There are other such non-linear mapping algorithms, such as t-SNE (t-distributed stochastic neighbor embedding) and Kohonen’s mapping (SOM, self-organizing map).

Ever since Mehta and Schwab laid out the relationship between restricted Boltzmann machines (RBM) and deep learning mathematically (see my previous entry), scientists have been discussing why deep learning works so well. Recently, Henry Lin and Max Tegmark put a preprint on arXiv (arXiv:1609.09225), arguing that deep learning works because it captures a few essential physical laws and properties. Tegmark is a cosmologist.

Physical laws are simple in a way that a few properties, such as locality, symmetry, hierarchy etc., lead to large-scale, universal, and often complex phenomena. A lot of machine learning algorithms, including deep learning algorithms, have deep relations with formalisms outlined in statistical mechanics.

A lot of machine learning algorithms are basically probability theory. They outlined a few types of algorithms that seek various types of probabilities. They related the probabilities to Hamiltonians in many-body systems.

They argued why neural networks can approximate functions (polynomials) so well, giving a simple neural network performing multiplication. With central limit theorem or Jaynes’ arguments (see my previous entry), a lot of multiplications, they said, can be approximated by low-order polynomial Hamiltonian. This is like a lot of many-body systems that can be approximated by 4-th order Landau-Ginzburg-Wilson (LGW) functional.

Properties such as locality reduces the number of hyper-parameters needed because it restricts to interactions among close proximities. Symmetry further reduces it, and also computational complexities. Symmetry and second order phase transition make scaling hypothesis possible, leading to the use of the tools such as renormalization group (RG). As many people have been arguing, deep learning resembles RG because it filters out unnecessary information and maps out the crucial features. Tegmark use classifying cats vs. dogs as an example, as in retrieving temperatures of a many-body systems using RG procedure. They gave a counter-example to Schwab’s paper with the probabilities cannot be preserved by RG procedure, but while it is sound, but it is not the point of the RG procedure anyway.

They also discussed about the no-flattening theorems for neural networks.

There are many tasks that involve coding, for example, putting kids into groups according to their age, labeling the webpages about their kinds, or putting students in Hogwarts into four colleges… And researchers or lawyers need to code people, according to their filled-in information, into occupations. Melissa Friesen, an investigator in Division of Cancer Epidemiology and Genetics (DCEG), National Cancer Institute (NCI), National Institutes of Health (NIH), saw the need of large-scale coding. Many researchers are dealing with big data concerning epidemiology. She led a research project, in collaboration with Office of Intramural Research (OIR), Center for Information Technology (CIT), National Institutes of Health (NIH), to develop an artificial intelligence system to cope with the problem. This leads to a publicly available tool called SOCcer, an acronym for “Standardized Occupation Coding for Computer-assisted Epidemiological Research.” (URL: http://soccer.nci.nih.gov/soccer/)

The system was initially developed in an attempt to find the correlation between the onset of cancers and other diseases and the occupation. “The application is not intended to replace expert coders, but rather to prioritize which job descriptions would benefit most from expert review,” said Friesen in an interview. She mainly works with Daniel Russ in CIT.

SOCcer takes job title, industry codes (in terms of SIC, Standard Industrial Classification), and job duties, and gives an occupational code called SOC 2010 (Standard Occupational Classification), used by U. S. federal government agencies. The data involves short text, often messy. There are 840 codes in SOC 2010 systems. Conventional natural language processing (NLP) methods may not apply. Friesen, Russ, and Kwan-Yuet (Stephen) Ho (also in OIR, CIT; a CSRA staff) use fuzzy logic, and maximum entropy (maxent) methods, with some feature engineering, to build various classifiers. These classifiers are aggregated together, as in stacked generalization (see my previous entry), using logistic regression, to give a final score.

SOCcer has a companion software, called SOCAssign, for expert coders to prioritize the codings. It was awarded with DCEG Informatics Tool Challenge 2015. SOCcer itself was awarded in 2016. And the SOCcer team was awarded for Scientific Award of Merit by CIT/OCIO in 2016 as well (see this). Their work was published in Occup. Environ. Med. The topic of word embedding algorithms has been one of the interests of this blog, as in this entry, with Word2Vec [Mikilov et. al. 2013] as one of the main examples. It is a great tool for text mining, (for example, see [Czerny 2015],) as it reduces the dimensions needed (compared to bag-of-words model). As an algorithm borrowed from computer vision, a lot of these algorithms use deep learning methods to train the model, while it was not exactly sure why it works. Despite that, there are many articles talking about how to train the model. [Goldberg & Levy 2014, Rong 2014 etc.] Addition and subtraction of the word vectors show amazing relationships that carry semantic values, as I have shown in my previous blog entry. [Ho 2015]

However, Tomas Mikolov is no longer working in Google, making the development of this algorithm discontinued. As a follow-up of their work, Stanford NLP group later proposed a model, called GloVe (Global Vectors), that embeds word vectors using probabilistic methods. [Pennington, Socher & Manning 2014] It can be implemented in the package glove-python in python, and text2vec in R (or see their CRAN post).  Their paper is neatly written, and a highly recommended read.

To explain the theory of GloVe, we start with some basic probabilistic picture in basic natural language processing (NLP). We suppose the relation between the words occur in certain text windows within a corpus, but the details are not important here. Assume that $i$, $j$, and $k$ are three words, and the conditional probability $P_{ik}$ is defined as $P_{ij} = P(j | i) = \frac{X_{ij}}{X_i}$,

where $X$‘s are the counts, and similarly for $P_{jk}$. And we are interested in the following ratio: $F(w_i, w_j, \tilde{w}_k) = \frac{P_{ik}}{P_{jk}}$.

The tilde means “context,” but we will later assume it is also a word. Citing the example from their paper, take $i$ as ice, and $j$ as steam. if $k$ is solid, then the ratio is expected to be large; or if $k$ is gas, then it is expected to be low. But if $k$ is water, which are related to both, or fashion, which is related to none, then the ratio is expected to be approximately 1.

And the addition and subtraction in Word2Vec is similar to this. We want the ratio to be like the subtraction as in Word2Vec (and multiplication as in addition), then we should modify the function $F$ such that, $F(w_i - w_j, \tilde{w}_k) = \frac{P_{ik}}{P_{jk}}$.

On the other hand, the input arguments of $F$ are vectors, but the output is a scalar. We avoid the issue by making the input argument as a dot product, $F( (w_i - w_j)^T \tilde{w}_k) = \frac{P_{ik}}{P_{jk}}$.

In NLP, the word-word co-occurrence matrices are symmetric, and our function $F$ should also be invariant under switching the labeling. We first require $F$ is be a homomorphism, $F((w_i - w_j)^T \tilde{w}_k) = \frac{F(w_i^T \tilde{w}_k) }{ F(w_j^T \tilde{w}_k)}$,

where we define, $F(w_i^T \tilde{w}_k) = P_{ik} = \frac{X_{ik}}{X_i}$.

It is clear that $F$ is an exponential function, but to ensure symmetry, we further define: $w_i^T \tilde{w}_k + b_i + \tilde{b}_k = \log X_{ik}$.

As a result of this equation, the authors defined the following cost function to optimize for GloVe model: $J = \sum_{i, j=1}^V f(X_{ij}) \left( w_i^T \tilde{w}_j + b_i + \tilde{b}_j - \log X_{ik} \right)^2$,

where $w_j$, $\tilde{w}_j$, $b_i$, and $\tilde{b}_j$ are parameters to learn. $f(x)$ is a weighting function. (Refer the details to the paper.) [Pennington, Socher & Manning 2014]

As Radim Řehůřek said in his blog entry, [Řehůřek 2014] it is a neat paper, but their evaluation is crappy.

This theory explained why certain similar relations can be achieved, such as Paris – France is roughly equal to Beijing – China, as both can be transformed to the ratio in the definition of $F$ above.

It is a neat paper, as it employs optimization theory and probability theory, without any dark box deep learning.

Embedding has been hot in recent years partly due to the success of Word2Vec, (see demo in my previous entry) although the idea has been around in academia for more than a decade. The idea is to transform a vector of integers into continuous, or embedded, representations. Keras, a Python package that implements neural network models (including the ANN, RNN, CNN etc.) by wrapping Theano or TensorFlow, implemented it, as shown in the example below (which converts a vector of 200 features into a continuous vector of 10):

from keras.layers import Embedding
from keras.models import Sequential

# define and compile the embedding model
model = Sequential()
model.compile('rmsprop', 'mse')  # optimizer: rmsprop; loss function: mean-squared error


We can then convert any features from 0 to 199 into vectors of 20, as shown below:

import numpy as np

model.predict(np.array([10, 90, 151]))


It outputs:

array([[[ 0.02915354,  0.03084954, -0.04160764, -0.01752155, -0.00056815,
-0.02512387, -0.02073313, -0.01154278, -0.00389587, -0.04596512]],

[[ 0.02981793, -0.02618774,  0.04137352, -0.04249889,  0.00456919,
0.04393572,  0.04139435,  0.04415271,  0.02636364, -0.04997493]],

[[ 0.00947296, -0.01643104, -0.03241419, -0.01145032,  0.03437041,
0.00386361, -0.03124221, -0.03837727, -0.04804075, -0.01442516]]])


Of course, one must not omit a similar algorithm called GloVe, developed by the Stanford NLP group. Their codes have been wrapped in both Python (package called glove) and R (library called text2vec).

Besides Word2Vec, there are other word embedding algorithms that try to complement Word2Vec, although many of them are more computationally costly. Previously, I introduced LDA2Vec in my previous entry, an algorithm that combines the locality of words and their global distribution in the corpus. And in fact, word embedding algorithms with a similar ideas are also invented by other scientists, as I have introduced in another entry.

However, there are word embedding algorithms coming out. Since most English words carry more than a single sense, different senses of a word might be best represented by different embedded vectors. Incorporating word sense disambiguation, a method called sense2vec has been introduced by Trask, Michalak, and Liu. (arXiv:1511.06388). Matthew Honnibal wrote a nice blog entry demonstrating its use.

There are also other related work, such as wang2vec that is more sensitive to word orders.

Big Bang Theory (Season 2, Episode 5): Euclid Alternative

DMV staff: Application?
Sheldon: I’m actually more or a theorist.

Note: feature image taken from Big Bang Theory (CBS). On October 14, 2015, I attended the regular meeting of the DCNLP meetup group, a group on natural language processing (NLP) in Washington, DC area. The talk was titled “Deep Learning for Question Answering“, spoken by Mr. Mohit Iyyer, a Ph.D. student in Department of Computer Science, University of Maryland (my alma mater!). He is a very good speaker.

I have no experience on deep learning at all although I did write a blog post remotely related. I even didn’t start training my first neural network until the next day after the talk. However, Mr. Iyyer explained what recurrent neural network (RNN), recursive neural network, and deep averaging network (DAN) are. This helped me a lot in order to understanding more about the principles of the famous word2vec model (which is something I am going to write about soon!). You can refer to his slides for more details. There are really a lot of talents in College Park, like another expert, Joe Yue Hei Ng, who is exploiting deep learning a lot as well.

The applications are awesome: with external knowledge to factual question answering, reasoning-based question answering, and visual question answering, with increasing order of challenging levels.

Mr. Iyyer and the participants discussed a lot about different packages. Mr. Iyyer uses Theano, a Python package for deep learning, which is good for model building and other analytical work. Some prefer Caffe. Some people, who are Java developers, also use deeplearning4j.

This meetup was a sacred one too, because it is the last time it was held in Stetsons Famous Bar & Grill at U Street, which is going to permanently close on Halloween this year. The group is eagerly looking for a new venue for the upcoming meetup. This meeting was a crowded one. I sincerely thank the organizers, Charlie Greenbacker and Liz Merkhofer, for hosting all these meetings, and Chris Phipps (a linguist from IBM Watson) for recording. There is no doubt that everyone who are in the so-called big data industry must know some statistics. However, statistics means differently to different peoples.

Statistics is an old field that was developed in the 18th century. In those times, people were urged to make conclusions out of a vast amount of data which were virtually not available, or were very costly to obtain. For example, someone wanted to know the average salary of the whole population, which required the census staff to survey the information from everyone in the population. It was something expensive to do in the old days. Therefore, sampling techniques were devised, and the wanted quantities can be estimated using an appropriate statistic.

Or when the scientists performed an experiment, even one data point costs a few million dollars. The experiments had to be designed in a way that the scientists extract the wanted information by looking at a few data points.

Or in testing some hypotheses, one needs to know only how to accept or reject a hypothesis using the statistical information available.

Hence, the traditional statistics is a body of knowledge that deduce the information of a whole population from a limited amount of data from a sample.

Theoretical Statistical Physics

There is a branch in physics called statistical physics, which originated from the 19th century. Later it became useful since Albert Einstein published its paper on Brownian motion in 1905. And now the methods in statistical physics is not only applied in solid state physics or condensed matter physics, but also in biophysics (e.g., diffusion), econophysics (e.g., the fairness and wealth distribution, see this previous blog post), and quantitative finance (e.g., binomial model, and its relation with Black-Scholes equation).

The techniques involved in statistical physics includes is the knowledge of probability theory and stochastic calculus (such as Ito calculus). Of course, it is how entropy, a concept from thermodynamics, entered probability theory and information theory. Extracted quantity are mostly expectation values and correlations, which are of interest to theorists.

This is very different from traditional statistics. When people know that I am a statistical physicist, they expect me to be familiar with t-test, which is not really the case. (Very often I have to look up every time I used them.)

Statistics in the Computing World

Unlike in traditional statistics or statistical physics, nowadays, we often get the statistical information directly from a vast amount of available data, thanks to the advance of technology and the reducing cost to access the technology. You can easily calculate the average salary of a population by a single command line on R or Python. Hence, statistics is no longer about extracting information from a limited amount of data, but a vast amount of data.

On the other hand, mathematical modeling is still important, but in a different sense. Models in statistical physics describes the world, but in information retrieval, models are built according to what we need.

P.S.: Philipp Janert wrote something similar in his Chapter 10 (“What You Really Need to Know About Classical Statistics”) in his “Data Analysis Using Open Source Tools“:

The basic statistical methods that we know today were developed in the late 19th and early 20th centuries, mostly in Great Britain, by a very small group of people. Of those, one worked for the Guinness brewing company and another—the most influential one of them—worked at an agricultural research lab (trying to increase crop yields and the like). This bit of historical context tells us something about their working conditions and primary challenges.

No computational capabilities All computations had to be performed with paper and pencil.

No graphing capabilities, either All graphs had to be generated with pencil, paper, and a ruler. (And complicated graphs—such as those requiring prior transformations or calculations using the data—were especially cumbersome.)

Very small and very expensive data sets Data sets were small (often not more than four to five points) and could be obtained only with great difficulty. (When it always takes a full growing season to generate a new data set, you try very hard to make do with the data you already have!)

In other words, their situation was almost entirely the opposite of our situation today:

• Computational power that is essentially free (within reason)
• Interactive graphing and visualization capabilities on every desktop
• Often huge amounts of data

It should therefore come as no surprise that the methods developed by those early researchers seem so out of place to us: they spent a great amount of effort and ingenuity solving problems we simply no longer have! This realization goes a long way toward explaining why classical statistics is the way it is and why it often seems so strange to us today.

P.S.: The graph at the beginning of this blog entry was plotted in Mathematica, by running the following:

Plot[Evaluate@Table[PDF[MaxwellDistribution[σ], x], {σ, {1, 2, 3}}], {x, 0, 10}, Filling -> Axis]