Class-specific soft voting based multiple extreme learning
machines ensemble
Jingjing Cao
a,b
, Sam Kwong
a,
n
, Ran Wang
a
, Xiaodong Li
a
,KeLi
a
, Xiangfei Kong
a
a
Department of Computer Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, SAR 999077, Hong Kong
b
School of Logistics Engineering, Wuhan University of Technology, Wuhan 430070, China
article info
Article history:
Received 7 August 2013
Received in revised form
5 December 2013
Accepted 3 February 2014
Available online 16 September 2014
Keywords:
Extreme learning machine
Soft voting
Condition number
Sparse ensemble
abstract
Compared with conventional weighted voting methods, class-specific soft voting (CSSV) system has
several advantages. On one hand, it not only deals with the soft class probability outputs but also refines
the weights from classifiers to classes. On the other hand, the class-specific weights can be used to
improve the combinative performance without increasing much computational load. This paper
proposes two weight optimization based ensemble methods (CSSV-ELM and SpaCSSV-ELM) under the
framework of CSSV scheme for multiple extreme learning machines (ELMs). The designed two models
are in terms of accuracy and sparsity aspects, respectively. Firstly, CSSV-ELM takes advantage of the
condition number of matrix, which reveals the stability of linear equation, to determine the weights of
base ELM classifiers. This model can reduce the unreliability induced by randomly input parameters of
a single ELM, and solve the ill-conditioned problem caused by linear system structure of ELM
simultaneously. Secondly, sparse ensemble methods can lower memory requirement and speed up
the classification process, but only for classifier-specific weight level. Therefore, a SpaCSSV-ELM method
is proposed by transforming the weight optimization problem to a sparse coding problem, which uses
the sparse representation technique for maintaining classification performance with less nonzero weight
coefficients. Experiments are carried out on twenty UCI data sets and Finance event series data and the
experimental results show the superior performance of the CSSV based ELM algorithms by comparing
with the state-of-the-art algorithms.
& 2014 Elsevier B.V. All rights reserved.
1. Introduction
Extreme learning machine (ELM) becomes popular for solving
classification problem due to its light computational requirements.
It is an extension of the single-hidden layer feedforward networks
(SLFNs). By making use of a least-square method, it analytically
obtains the output weights of SLFNs [1]. Moreover, ELM emphasizes
on achieving both the smallest norm of output weights and the
least training error, which is different from conventional neural type
of SLFNs. Essentially, ELM is originally designed by utilizing random
computational nodes, which are independent of the training data.
The process for tuning the hidden layer parameters is avoided,
which significantly shortens the learning time. A great many of ELM
based algorithms have been done in recent years [1–3].
However, since the input hidden nodes are randomly generated,
it is easy to misclassify patterns that are close to the boundary [3,4].
In order to improve the classification performance, a number of real
world applications based on ensemble learning have been done in
previous research [2,5–7]. Different from designing a single classi-
fier in traditional pattern recognition field, ensemble learning
consists of a group of machine learning algorithms that aims at
constructing multiple classifiers to form a hybrid predictive model.
Generally speaking, the overall classification performance of ensem-
ble classifier could be better than using a single classifier. The
ensemble learning aims at a high accurate prediction at the expense
of increased complexity. In multiple classifier system (MCS), the
field of ensemble learning usually employs homogeneous base
learners. In the past few decades, many ensemble techniques
[8–10] are proposed to enhance the reliability of multiple models.
Besides, ensemble methods are also successfully applied into
applications from a wide range of fields [11–13] due to their
remarkable capability in increasing the classification performance
of a learning model.
Numerous works have been proposed regarding to ensemble
ELMs in recent years. In [14], Liang et al. proposed the online
sequential extreme learning machine (OS-ELM), which shows better
generalization behavior than the other sequential algorithms. Then
in [5], Lan et al. extended OS-ELM to an ensemble version and
improved the stability. In [6], Liu and Wang pointed out that ELM
might be prone to overfit since it approximates the training data in
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journal homepage: www.elsevier.com/locate/neucom
Neurocomputing
http://dx.doi.org/10.1016/j.neucom.2014.02.072
0925-2312/& 2014 Elsevier B.V. All rights reserved.
n
Corresponding author.
E-mail address: cssamk@cityu.edu.hk (S. Kwong).
Neurocomputing 149 (2015) 275–284