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layout: post title: "Machine Learning笔记 第10周" date: "2016-03-27 23:27:45" categories: 计算机科学 excerpt: "第十周根本没时间上课，只能利用第11周的春假补全。 This week: going over Feature Transformation t..."

auth: conge

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第十周根本没时间上课，只能利用第11周的春假补全。

This week: going over Feature Transformation this week, and starting on Information Theory.

• Feature selection is a subset of feature transformation
• Transformation operator is linear combinations of original features

Why do Feature Transformation

• XOR, Kernel methods, Neural networks already do FT.
• ad hoc Information Retrieval Problem: finding documents within a corpus that are relevant to an information need specified using a query. (Query is unknown)
• Problems of Information Retrieval:
  • Polysemy: e.g. a word have multiple meanings; cause false positive problem
  • Synonymy: e.g. a meaning can be expressed by multiple words. can cause false negatives problems.

PCA

This paper does a fantastic job building the intuition and implementation behind PCA

An eigenproblem is a computational problem that can be solved by finding the eigenvalues and/or eigenvectors of a matrix. In PCA, we are analyzing the covariance matrix (see the paper for details)

PCA Features

• maximize variance
• mutually orthogonal (every components are perpendicular to each other)
• Global algorithm: the resulted components have a global constraint which is that they must be orthogonal

• it gives best reconstruction

• EigenValue monotonically not increasing and 0 eigenvalue = ignorable (irrelevant, maybe not useful).

• It's well studied and fast to run.

• it's like a classification. and using a filtering method to select dimensions to use.
• PCA is about finding

ICA

ICA has also been applied to the information retrieval problem, in a paper written by Charles himself

• find components that are statistically independent from each other using mutual information.
• Designed to solve the blind source separation problem.
• Model: given observables, find hidden variables.

• ICA is more suitable for BSS problems and is directional.
• Eg,
  • PCA on faces will separate image based on brightness and average faces. ICA will get features such as nose, mouth etc, which are basic components of a face.

Alternatives:

Random components Analysis: generates random directions

• Can project to smaller dimensions (m << n)but in practice often have more dimensions than PCA.
• Can project to higher dimensions (m > n)
• It works and works very fast.

• Linear Discriminant analysis: find a projection that discriminates based on the label

wrap up

This excellent paper is a great resource for the Feature Transformation methods from this course, and beyond

2016-03-17 初稿
2016-03-26 补完