unit 4 nearest neighbor.ppt

Nearest Neighbour Rule
Non-parametric pattern
classification.
Consider a two class problem
where each sample consists of
two measurements (x,y).
k = 1
k = 3
For a given query point q,
assign the class of the
nearest neighbour.
Compute the k nearest
neighbours and assign the
class by majority vote.

K-Nearest Neighbor
• When we interpret records as points in a data
space, we can define the concepts of
neighborhood.
– “Records that are close to each other live in each
others neighborhood.”
– “Records of the same type will be close to each other
in the data space.”
– i.e. customers of same type will show same behavior.

K-Nearest Neighbor
• Basic principle of K-nearest neighbor is
– “Do as your neighbor Do”
• If we want to predict the behavior of certain individual,
we first to look at the behavior of, for example: ten
individuals that are close to him/her in the data space.
• We calculate the average of 10 neighbors and this
average value will be the prediction for our individual.
• K=> No. of Neighbors. 5-Nearest neighbor => 5
neighbors to be taken for calculation

Nearest Neighbour Issues
• Expensive
– To determine the nearest neighbour of a query point q, must compute
the distance to all N training examples
+ Pre-sort training examples into fast data structures (kd-trees)
+ Compute only an approximate distance (LSH)
+ Remove redundant data (condensing)
• Storage Requirements
– Must store all training data P
+ Remove redundant data (condensing)
- Pre-sorting often increases the storage requirements
• High Dimensional Data
– “Curse of Dimensionality”
• Required amount of training data increases exponentially with dimension
• Computational cost also increases dramatically
• Partitioning techniques degrade to linear search in high dimension

Questions
• What distance measure to use?
– Often Euclidean distance is used
– Locally adaptive metrics
– More complicated with non-numeric data, or when different dimensions
have different scales
• Choice of k?
– Cross-validation
– 1-NN often performs well in practice
– k-NN needed for overlapping classes
– Re-label all data according to k-NN, then classify with 1-NN
– Reduce k-NN problem to 1-NN through dataset editing

Exact Nearest Neighbour
• Asymptotic error (infinite sample size) is less than twice the Bayes
classification error
– Requires a lot of training data
• Expensive for high dimensional data (d>20?)
• O(Nd) complexity for both storage and query time
– N is the number of training examples, d is the dimension of each sample
– This can be reduced through dataset editing/condensing

Decision Regions
Each cell contains one
sample, and every
location within the cell is
closer to that sample than
to any other sample.
A Voronoi diagram divides
the space into such cells.
Every query point will be assigned the classification of the sample within that
cell. The decision boundary separates the class regions based on the 1-NN
decision rule.
Knowledge of this boundary is sufficient to classify new points.
The boundary itself is rarely computed; many algorithms seek to retain only
those points necessary to generate an identical boundary.

Condensing
• Aim is to reduce the number of training samples
• Retain only the samples that are needed to define the decision boundary
• Decision Boundary Consistent – a subset whose nearest neighbour decision
boundary is identical to the boundary of the entire training set
• Minimum Consistent Set – the smallest subset of the training data that correctly
classifies all of the original training data
Original data Condensed data Minimum Consistent Set

Condensing
• Condensed Nearest Neighbour (CNN)
Hart 1968
– Incremental
– Order dependent
– Neither minimal nor decision
boundary consistent
– O(n3) for brute-force method
– Can follow up with reduced NN
[Gates72]
• Remove a sample if doing so
does not cause any incorrect
classifications
1. Initialize subset with a single
training example
2. Classify all remaining
samples using the subset,
and transfer any incorrectly
classified samples to the
subset
3. Return to 2 until no transfers
occurred or the subset is full

Proximity Graphs
• Condensing aims to retain points along the decision boundary
• How to identify such points?
– Neighbouring points of different classes
• Proximity graphs provide various definitions of “neighbour”
   
NNG MST RNG GG DT
NNG = Nearest Neighbour Graph
MST = Minimum Spanning Tree
RNG = Relative Neighbourhood Graph
GG = Gabriel Graph
DT = Delaunay Triangulation

Proximity Graphs: Delaunay
• The Delaunay Triangulation is the dual of the
Voronoi diagram
• Three points are each others neighbours if their
tangent sphere contains no other points
• Voronoi editing: retain those points whose
neighbours (as defined by the Delaunay
Triangulation) are of the opposite class
• The decision boundary is identical
• Conservative subset
• Retains extra points
• Expensive to compute in high
dimensions

Proximity Graphs: Gabriel
• The Gabriel graph is a subset of the
Delaunay Triangulation
• Points are neighbours only if their
(diametral) sphere of influence is
empty
• Does not preserve the identical
decision boundary, but most changes
occur outside the convex hull of the
data points
• Can be computed more efficiently
Green lines denote
“Tomek links”

Proximity Graphs: RNG
• The Relative Neighbourhood Graph (RNG)
is a subset of the Gabriel graph
• Two points are neighbours if the “lune”
defined by the intersection of their radial
spheres is empty
• Further reduces the number of neighbours
• Decision boundary changes are often
drastic, and not guaranteed to be training
set consistent
Gabriel edited RNG edited – not consistent

Dataset Reduction: Editing
• Training data may contain noise, overlapping classes
– starting to make assumptions about the underlying distributions
• Editing seeks to remove noisy points and produce smooth decision
boundaries – often by retaining points far from the decision boundaries
• Results in homogenous clusters of points

Wilson Editing
• Wilson 1972
• Remove points that do not agree with the majority of their k nearest neighbours
Wilson editing with k=7
Original data
Earlier example
Wilson editing with k=7
Original data
Overlapping classes

Multi-edit
• Multi-edit [Devijer & Kittler ’79]
– Repeatedly apply Wilson editing
to random partitions
– Classify with the 1-NN rule
• Approximates the error rate of the
Bayes decision rule
1. Diffusion: divide data into N ≥
3 random subsets
2. Classification: Classify Si
using 1-NN with S(i+1)Mod N as
the training set (i = 1..N)
3. Editing: Discard all samples
incorrectly classified in (2)
4. Confusion: Pool all remaining
samples into a new set
5. Termination: If the last I
iterations produced no editing
then end; otherwise go to (1)
Multi-edit, 8 iterations – last 3 same
Voronoi editing

Combined Editing/Condensing
• First edit the data to remove noise and smooth the boundary
• Then condense to obtain a smaller subset

unit 4 nearest neighbor.ppt

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