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Forests of Randomized Shapelet Trees
Stockholm University, Faculty of Social Sciences, Department of Computer and Systems Sciences.
Stockholm University, Faculty of Social Sciences, Department of Computer and Systems Sciences.
Stockholm University, Faculty of Social Sciences, Department of Computer and Systems Sciences.
2015 (English)In: Statistical Learning and Data Sciences: Proceedings / [ed] Alexander Gammerman, Vladimir Vovk, Harris Papadopoulos, Springer, 2015, 126-136 p.Conference paper, Published paper (Refereed)
Abstract [en]

Shapelets have recently been proposed for data series classification, due to their ability to capture phase independent and local information. Decision trees based on shapelets have been shown to provide not only interpretable models, but also, in many cases, state-of-the-art predictive performance. Shapelet discovery is however computationally costly, and although several techniques for speeding up the technique have been proposed, the computational cost is still in many cases prohibitive. In this work, an ensemble based method, referred to as Random Shapelet Forest (RSF), is proposed, which builds on the success of the random forest algorithm, and which is shown to have a lower computational complexity than the original shapelet tree learning algorithm. An extensive empirical investigation shows that the algorithm provides competitive predictive performance and that a proposed way of calculating importance scores can be used to successfully identify influential regions.

Place, publisher, year, edition, pages
Springer, 2015. 126-136 p.
Series
Lecture Notes in Computer Science, ISSN 0302-9743 ; 9047
Keyword [en]
data series classification, shapelets, decision trees, ensemble
National Category
Information Systems
Research subject
Computer and Systems Sciences
Identifiers
URN: urn:nbn:se:su:diva-122820DOI: 10.1007/978-3-319-17091-6_8ISBN: 978-3-319-17091-6 (print)OAI: oai:DiVA.org:su-122820DiVA: diva2:868623
Conference
Third International Symposium, SLDS 2015, Egham, UK, April 20-23, 2015
Available from: 2015-11-11 Created: 2015-11-10 Last updated: 2017-04-28Bibliographically approved
In thesis
1. Order in the random forest
Open this publication in new window or tab >>Order in the random forest
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In many domains, repeated measurements are systematically collected to obtain the characteristics of objects or situations that evolve over time or other logical orderings. Although the classification of such data series shares many similarities with traditional multidimensional classification, inducing accurate machine learning models using traditional algorithms are typically infeasible since the order of the values must be considered.

In this thesis, the challenges related to inducing predictive models from data series using a class of algorithms known as random forests are studied for the purpose of efficiently and effectively classifying (i) univariate, (ii) multivariate and (iii) heterogeneous data series either directly in their sequential form or indirectly as transformed to sparse and high-dimensional representations. In the thesis, methods are developed to address the challenges of (a) handling sparse and high-dimensional data, (b) data series classification and (c) early time series classification using random forests. The proposed algorithms are empirically evaluated in large-scale experiments and practically evaluated in the context of detecting adverse drug events.

In the first part of the thesis, it is demonstrated that minor modifications to the random forest algorithm and the use of a random projection technique can improve the effectiveness of random forests when faced with discrete data series projected to sparse and high-dimensional representations. In the second part of the thesis, an algorithm for inducing random forests directly from univariate, multivariate and heterogeneous data series using phase-independent patterns is introduced and shown to be highly effective in terms of both computational and predictive performance. Then, leveraging the notion of phase-independent patterns, the random forest is extended to allow for early classification of time series and is shown to perform favorably when compared to alternatives. The conclusions of the thesis not only reaffirm the empirical effectiveness of random forests for traditional multidimensional data but also indicate that the random forest framework can, with success, be extended to sequential data representations.

Place, publisher, year, edition, pages
Stockholm: Department of Computer and Systems Sciences, Stockholm University, 2017. 76 p.
Series
Report Series / Department of Computer & Systems Sciences, ISSN 1101-8526 ; 17-004
Keyword
Machine learning, random forest, ensemble, time series, data series, sequential data, sparse data, high-dimensional data
National Category
Computer and Information Science
Research subject
Computer and Systems Sciences
Identifiers
urn:nbn:se:su:diva-142052 (URN)978-91-7649-827-9 (ISBN)978-91-7649-828-6 (ISBN)
Public defence
2017-06-08, L30, NOD-huset, Borgarfjordsgatan 12, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , IIS11-0053
Available from: 2017-05-16 Created: 2017-04-24 Last updated: 2017-05-15Bibliographically approved

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