Visualization and Navigation of Document Information Spaces Using a Self-Organizing Map

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Visualization and Navigation of Document Information Spaces Using a Self-Organizing Map

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Visualization and Navigation of Document Information Spaces Using a Self-Organizing Map

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Visualization and Navigation of Document Information Spaces Using a Self-Organizing Map

Daniel X. Pape

Community Architectures for Network Information Systems

dpape@canis.uiuc.edu

www.canis.uiuc.edu

CSNA’98

6/18/98

- Self-Organizing Map (SOM) Algorithm
- U-Matrix Algorithm for SOM Visualization
- SOM Navigation Application
- Document Representation and Collection Examples
- Problems and Optimizations
- Future Work

- Input
- Number (n) of Feature Vectors (x)
- format:
vector name: a, b, c, d

- examples:
1: 0.1, 0.2, 0.3, 0.4

2: 0.2, 0.3, 0.3, 0.2

- Output
- Neural network Map of (M) Nodes
- Each node has an associated Weight Vector (m) of the same dimensionality as the input feature vectors
- Examples:
m1: 0.1, 0.2, 0.3, 0.4

m2: 0.2, 0.3, 0.3, 0.2

- Output (cont.)
- Nodes laid out in a grid:

- Other Parameters
- Number of timesteps (T)
- Learning Rate (eta)

SOM() {

foreach timestep t {

foreach feature vector fv {

wnode = find_winning_node(fv)

update_local_neighborhood(wnode)

}

}

}

find_winning_node() {

foreach node n {

compute distance of m to feature vector

}

return node with the smallest distance

}

update_local_neighborhood(wnode) {

foreach node n {

m = m + eta [x - m]

}

}

- Provides a simple way to visualize cluster boundaries on the map
- Simple algorithm:
- for each node in the map, compute the average of the distances between its weight vector and those of its immediate neighbors

- Average distance is a measure of a node’s similarity between it and its neighbors

- Interpretation
- one can encode the U-Matrix measurements as greyscale values in an image, or as altitudes on a terrain
- landscape that represents the document space: the valleys, or dark areas are the clusters of data, and the mountains, or light areas are the boundaries between the clusters

- Example:
- dataset of random three dimensional points, arranged in four obvious clusters

Four (color-coded) clusters of three-dimensional points

Oblique projection of a terrain derived from the U-Matrix

Terrain for a real document collection

- Feature vectors are encoded as 0’s and 1’s
- Weight vectors have real values from 0 to 1
- Sort weight vector dimensions by element value
- dimension with greatest value is “best” noun phrase for that node

- Aggregate nodes with the same “best” noun phrase into groups

- 3D Space-Flight
- Hierarchical Navigation

- Noun phrases extracted
- Set of unique noun phrases computed
- each noun phrase becomes a dimension of the data set

- Each document represented by a binary vector with a 1 or a 0 denoting the existence or absence of each noun phrase

- Example:
- 10 total noun phrases:
alexander, king, macedonians, darius, philip, horse, soldiers, battle, army, death

- each element of the feature vector will be a 1 or a 0:
- 1: 1, 1, 0, 0, 1, 1, 0, 0, 0, 0
- 2: 0, 1, 0, 1, 0, 0, 1, 1, 1, 1

- 10 total noun phrases:

- As document sets get larger, the feature vectors get longer, use more memory, etc.
- Execution time grows to unrealistic lengths

- Need algorithm refinements for sparse feature vectors
- Need a faster way to do the find_winning_node() computation
- Need a better way to do the update_local_neighborhood() computation

- Intelligent support for sparse feature vectors
- saves on memory usage
- greatly improves speed of the weight vector update computation

- SOM weight vectors become partially ordered very quickly

U-Matrix Visualization of an Initial, Unordered SOM

Partially Ordered SOM after 5 timesteps

- Don’t do a global search for the winner
- Start search from last known winner position
- Pro:
- usually finds a new winner very quickly

- Con:
- this new search for a winner can sometimes get stuck in a local minima

- Nodes get told to “update” quite often
- Weight vector is made public only during a find_winner() search
- With local find_winning_node() search, a lazy neighborhood weight vector update can be performed

- Cache update requests
- each node will store the winning node and feature vector for each update request

- The node performs the update computations called for by the stored update requests only when asked for its weight vector
- Possible reduction of number of requests by averaging the feature vectors in the cache

- Parallelization
- Label Problem

- Current Procedure not very good
- Cluster boundaries
- Term selection

- Image processing
- Geometric

- Image processing example:

- Too many unique noun phrases
- Too many dimensions in the feature vector data

- “Knee” of frequency curve