Inspire…Educate…Transform.
Self Organized Maps
Lt. Suryaprakash Kompalli
Senior Mentor, International School of
Engineering
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Self organizing maps
• Invented by Teuvo Kohonen, a professor of
the Academy of Finland.
• Kohonen described SOM as a visualization
and analysis tool kit for high dimensional
data.
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• SOMS are however used for clustering,
dimensionality reduction and
classification.
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The Basics
http://ssdi.di.fct.unl.pt/aadm/aadm1011/slides/LectureSOM.pdf
Sample
Best matching unit
Original nodes
Nodes in weight spac
x1 x2 x3 ……. xd
• Several nodes arranged in a grid at uniform distance.
• Weight from each input (1…d) to node (1…n)
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• Training modifies weights:
– Similar inputs activate neighboring nodes
– After training, distance between weights indicates how close the
nodes are
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BMU
• Which of the two nodes 1:(0.1, 0.2, 0.3) and 2:
(0.9, 0.1, 0.1) are closer to the input vector I=(1,
0, 0).
• Distance1 = sqrt( (1 - 0.1)2 + (0 - 0.2)2+ (0 -
0.3)2 ) = 0.96
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Distance2= 0.17
Hence, BMU is 2
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Training?
•
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Training?
•
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SOM Parameters
http://ssdi.di.fct.unl.pt/aadm/aadm1011/slides/LectureSOM.pdf
Classical vs Batch
Weights updated at each sample
Weights updated at each epoch
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How to use SOM?
Pre-process data: Generate
Standardization Build Self heatmaps and
Remove invalids Organizing Maps clusters to
Remove outliers interpret data
• In a random class, if students with similar
features sit as close as possible to each
other
– Age, grades, hobbies etc
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• If you examine their grades with respect
to new positions, this will be a SOM
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SOMs in Practice
• Example from a tutorial by Shane Lynn
– Other examples:
• http
://www.r-bloggers.com/self-organising-maps-for-customer-segmentation-using
-r
/
• http://
manuals.bioinformatics.ucr.edu/home/R_BioCondManual#clustering_primer
• Census data from Dublin:
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– ~4000 localities
– Average values for: age, household size,
education, car ownership,
– Percentage values for: health, rent,
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SOM on Census Data
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Number of data points mapping to each node.
Many blues/reds indicate too large/small model
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SOM on Census Data
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Interesting patterns can be seen
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SOM on Census Data
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Heat map of average education level
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SOM on Census Data
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SOM on Census Data
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Class to take 15 minute break –
run SOM code and suggest other
comparisons
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SOM on Census Data
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SOM on Census Data
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Clusters on the map of Dublin
Cluster Nodes using Hierarchical clustering
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References for Kohonen SOMs
• Online tutorials:
– https://www.youtube.com/watch?v=LjJeT7rwvF4
– http://www.cs.bham.ac.uk/~jxb/INC/l16.pdf
– http://www.r-bloggers.com/self-organising-maps-for-customer-se
gmentation-using-r
/
– http://ssdi.di.fct.unl.pt/aadm/aadm1011/slides/LectureSOM.pdf
– http://
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www.academia.edu/11322466/Using_R_to_Map_Crime_Density_a
nd_Demographics_in_Boston_from_2012-2014
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