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# LIST OF ABBREVIATIONS AND GLOSSARY OF TERMS

## Abbreviations

```
EM 		 expectation maximisation
FA 		 factor analysis
GTM 		 generative topographic mapping
ICA 		 independent component analysis
IFA 		 independent factor analysis
MAP 		 maximum a posteriori
MDL 		 minimum description length
ML 		 maximum likelihood
MLP 		 multi-layer perceptron
PCA 		 principal component analysis
SOM 		 self-organising map
ST 		 signal transformation
```

## Glossary of terms

artificial neural network:
A model which consists of simple building-blocks. The development of such models has been inspired by neurobiological findings. The building-blocks are termed neurons in analogy to biological brain.

auto-associative learning:
Representation of the observations is learned by finding a mapping from observations to themselves through an ``information bottleneck'' which forces the model to produce a compact coding of the observations. The recognition model and generative model are learned simultaneously.

Bayesian probability theory:
In Bayesian probability theory, probability is a measure of subjective belief as opposed to frequentist statistics where probability is interpreted as the relative frequency of occurrences in an infinite sequence of trials.

factor:
In generative models, the regularities in the observations are assumed to have been caused by underlying factors, also termed hidden causes, latent variables or sources.

factor analysis:
A technique for finding a generative model which can represent some of the statistical structure of the observations. Usually refers to linear factor analysis where the generative model is linear.

feature:
Feature describes a relevant aspect of observations. The term is often used in connection with recognition models. Bears resemblance to the term factor which is more common in connection to generative models.

ensemble learning:
A technique for approximating the exact application of Bayesian probability theory.

generative model:
A model which explicitly states how the observations are assumed to have been generated. See recognition model.

graphical model:
A graphical representation of the causal structure of a probabilistic model. Variables are denoted by circles and arrows are used for representing the conditional dependences.

hidden cause:
See factor.

latent variable:
See factor.

posterior probability:
Expresses the beliefs after making an observation. Sometimes referred to as the posterior.

prior probability:
Expresses the beliefs before making an observation. Sometimes referred to as the prior.

probability density:
Any single value of a continuous valued variable usually has zero probability and only a finite range of values has a nonzero probability. Probability of a continuous variable can be characterised by probability density which is defined to be the probability of a range divided by the size of the range.

probability mass:
In analogy to physical mass and density, ordinary probability can be called probability mass in order to distinguish it from probability density.

recognition model:
A model which states how features can be obtained from the observations. See generative model.

signal transformation approach:
Finds a recognition model by optimising a given criterion over the resulting features.

source:
See factor.

supervised learning:
Aims at building a model which can mimic the responses of a ``teacher'' who provides two sets of observations: inputs and the corresponding desired outputs. See unsupervised learning.

unsupervise learning:
The goal in unsupervised learning is to find an internal representation of the statistical structure of the observations. See supervised learning.

volume:
In analogy to physical mass, density and volume, the size of range of continuous valued variables can be called volume. See probability density.

Harri Valpola
2000-10-31