Archive for the ‘Machine Learning’ Category

Regularized Least Squares

Wednesday, February 9th, 2011

Recently, I’ve contributed a bunch of improvements (sparse matrix support, classification, generalized cross-validation) to the ridge module in scikits.learn. Since I’m receiving good feedback regarding my posts on Machine Learning, I’m taking this as an opportunity to summarize some important points about Regularized Least Squares, and more precisely Ridge regression.

Kernel Perceptron in Python

Sunday, October 31st, 2010

The Perceptron (Rosenblatt, 1957) is one of the oldest and simplest Machine Learning algorithms. It’s also trivial to kernelize, which makes it an ideal candidate to gain insights on kernel methods.

Support Vector Machines in Python

Sunday, September 19th, 2010

Support Vector Machines (SVM) are the state-of-the-art classifier in many applications and have become ubiquitous thanks to the wealth of open-source libraries implementing them. However, you learn a lot more by actually doing than by just reading, so let’s play a little bit with SVM in Python! To make it easier to read, we will use the same notation as in Christopher Bishop’s book “Pattern Recognition and Machine Learning”.

Latent Dirichlet Allocation in Python

Saturday, August 21st, 2010

Like Latent Semantic Analysis (LSA) and probabilistic LSA (pLSA) – see my previous post “LSA and pLSA in Python“, Latent Dirichlet Allocation (LDA) is an algorithm which, given a collection of documents and nothing more (no supervision needed), can uncover the “topics” expressed by documents in that collection. LDA can be seen as a Bayesian extension of pLSA.

Semi-supervised Naive Bayes in Python

Monday, June 21st, 2010


The Expectation-Maximization (EM) algorithm is a popular algorithm in statistics and machine learning to estimate the parameters of a model that depends on latent variables. (A latent variable is a variable that is not expressed in the dataset and thus that you can’t directly count. For example, in pLSA, the document topics z are latent variables.) EM is very intuitive. It works by pretending that we know what we’re looking for: the model parameters. First, we make an initial guess, which can be either random or “our best bet”. Then, in the E-step, we use our current model parameters to estimate some “measures”, the ones we would have used to compute the parameters, had they been available to us. In the M-step, we use these measures to compute the model parameters. The beauty of EM is that by iteratively repeating these two steps, the algorithm will provably converge to a local maximum for the likelihood that the model generated the data.

LSA and pLSA in Python

Sunday, June 13th, 2010

Latent Semantic Analysis (LSA) and its probabilistic counterpart pLSA are two well known techniques in Natural Language Processing that aim to analyze the co-occurrences of terms in a corpus of documents in order to find hidden/latent factors, regarded as topics or concepts. Since the number of topics/concepts is usually greatly inferior to the number of words and since it is not necessary to know the document categories/classes, LSA and pLSA are thus unsupervised dimensionality reduction techniques. Applications include information retrieval, document classification and collaborative filtering.

Introduction to Dynamic Time Warping

Monday, August 31st, 2009

Recently, I’ve been working on a new handwriting recognition engine for Tegaki based on Dynamic Time Warping and I figured it would be interesting to make a short, informal introduction to it.

Dynamic Time Warping (DTW) is a well-known algorithm which aims at comparing and aligning two sequences of data points (a.k.a time series). Although it was originally developed for speech recognition (see [1]), it has also been applied to many other fields like bioinformatics, econometrics and, of course, handwriting recognition.

Character encoding detection

Sunday, August 17th, 2008

Two years ago, I wrote about a port to Ruby of Universal Encoding Detector, which is itself a port to Python of Mozilla’s character encoding detection algorithm.

Recently being interested in Machine Learning, I read about naive Bayes classifiers. I then remembered the encoding detector program and thought that naive Bayes classifiers would be a good candidate for this kind of problem. Going back to the Universal Encoding Detector’s home page, I found a link to:

A composite approach to language/encoding detection