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Machine Learning Notes 1

1) Machine learning (ML) related definitions

Definition A (Athur Samuel, 1959): Machine learning is the field of study which gives computers the ability to learn without being explicitly programmed.

Definition B (Tom Mitchell, 1998): Well-posed machine learning problem: A computer program is said to learn from experience E with respect to some task T and some performance measure P, if its performance on T, as measured by P, improves with experience E.

2) Types of ML algorithms

In general any ML algorithm can be classified into one of the following two categories

  • Supervised learning (the "right answers" are given in the initial data set; i.e. the data set is labeled)
    • Regression: trying to predict the values of some quantity which we view as a continuous function
    • Classification: trying to predict the values of some quantity whose possible values form a small finite discrete set (that set could be e.g. {Yes, No}; {0,1,2,3}; {Cat, Dog, Other}; etc.); trying to classify a set of data points into a small finite number of classes
  • Unsupervised learning (the "right answers" are not given in the initial data set; i.e. the data set is unlabeled); in unsupervised learning the task is to automatically find some structure in the unlabeled data set that is given
    • Clustering: a data set is given, the task is to break it down into several separate clusters (i.e. into several separate disjoint data sets). Examples: 1) Google news (news articles from various sources are grouped together if they are about the same story); 2) Market segmentation: find market segments/clusters in a dataset which contains data about all customers of a given company.
    • Non-clustering: 
      • Independent component analysis (used in signal processing). Example: separating different voices (or say different audio tracks) out of one (or a few) given chaotic audio recordings (see e.g. (1) cocktail party effect, (2) cocktail party problem and algorithm). The so-called "cocktail party algorithm" allows you to find structure in a chaotic environment (identifying individual voices and music from a mesh of sounds at a cocktail party).
      • Anomaly detection (detect unusual data points in a given dataset). Anomaly detection (also referred to as outlier detection and sometimes as novelty detection) is generally understood to be the identification of rare items, events or observations which deviate significantly from the majority of the data and do not conform to a well defined notion of normal behavior (Source: Wikipedia). 
      • Dimensionality reduction (compress data points using fewer numbers). Dimensionality reduction, or dimension reduction, is the transformation of data from a higher-dimensional space into a lower-dimensional space so that the lower-dimensional representation retains some meaningful properties of the original data, ideally close to its intrinsic dimension (Source: Wikipedia).
3) GNU Octave

This is a powerful framework for quick prototyping of solutions/algorithms for ML problems (see here: GNU Octave)

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