Acta Polytechnica Scandinavica Mathematics, Computing and Management in Engineering Series No. 82 Data Exploration Using Self-organizing Maps Data Exploration Using Self-organizing Maps. Acta Polytechnica

Finding structures in vast multidimensional data sets be they measurement data statistics or textual documents is di cult and time consuming Interesting novel relations between the data items may be hidden in the data The self organizing map SOM algorithm of Kohonen can be used to aid the exploration the structures in the data sets can be illustrated on special map displays In this work the methodology of using SOMs for exploratory data analysis or data mining is reviewed and developed further The properties of the maps are compared with the properties of related methods intended for visualizing high dimensional multivariate data sets In a set of case studies the SOM algorithm is applied to analyzing electroencephalograms to illustrating structures of the standard of living in the world and to organizing full text document collections Measures are proposed for evaluating the quality of di erent types of maps in representing a given data set and for measuring the robustness of the illustrations the maps produce The same measures may also be used for comparing the knowledge that di erent maps represent Feature extraction must in general be tailored to the application as is done in the case studies There exists however an algorithm called the adaptive subspace self organizing map recently developed by Kohonen which may be of help It extracts invariant features automatically from a data set The algorithm is here characterized in terms of an objective function and demonstrated to be able to identify input patterns subject to di erent transformations Moreover it could also aid in feature exploration the kernels that the algorithm creates to achieve invariance can be illustrated on map displays similar to those that are used for illustrating the data sets c All rights reserved No part of the publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording or otherwise without the prior written permission of the author