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Feature Selection for Unsupervised Machine Learning of Accelerometer Data Physical Activity Clusters – A Systematic Review

journal contribution
posted on 03.09.2021, 11:15 by Petra J Jones, Mike Catt, Melanie J Davies, Charlotte L Edwardson, Evgeny M Mirkes, Kamlesh Khunti, Tom Yates, Alex V Rowlands
Background
Identifying clusters of physical activity (PA) from accelerometer data is important to identify levels of sedentary behaviour and physical activity associated with risks of serious health conditions and time spent engaging in healthy PA. Unsupervised machine learning models can capture PA in everyday free-living activity without the need for labelled data. However, there is scant research addressing the selection of features from accelerometer data. The aim of this systematic review is to summarise feature selection techniques applied in studies concerned with unsupervised machine learning of accelerometer-based device obtained physical activity, and to identify commonly used features identified through these techniques. Feature selection methods can reduce the complexity and computational burden of these models by removing less important features and assist in understanding the relative importance of feature sets and individual features in clustering.

Method
We conducted a systematic search of Pubmed, Medline, Google Scholar, Scopus, Arxiv and Web of Science databases to identify studies published before January 2021 which used feature selection methods to derive PA clusters using unsupervised machine learning models.

Results
A total of 13 studies were eligible for inclusion within the review. The most popular feature selection techniques were Principal Component Analysis (PCA) and correlation-based methods, with k-means frequently used in clustering accelerometer data. Cluster quality evaluation methods were diverse, including both external (e.g. cluster purity) or internal evaluation measures (silhouette score most frequently). Only four of the 13 studies had more than 25 participants and only four studies included two or more datasets.

Conclusion
There is a need to assess multiple feature selection methods upon large cohort data consisting of multiple (3 or more) PA datasets. The cut-off criteria e.g. number of components, pairwise correlation value, explained variance ratio for PCA, etc. should be expressly stated along with any hyperparameters used in clustering.

History

Citation

Gait & Posture, Volume 90, October 2021, Pages 120-128

Author affiliation

Diabetes Research Centre, College of Life Sciences

Version

AM (Accepted Manuscript)

Published in

Gait & Posture

Volume

90

Pagination

120-128

Publisher

Elsevier BV

issn

0966-6362

Acceptance date

08/08/2021

Copyright date

2021

Available date

13/08/2022

Language

en

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