Abstract
Gradient based optimization methods often converge quickly to a local optimum. However, the check loss function used by quantile regression model is not everywhere differentiable, which prevents the gradient based optimization methods from being applicable. As such, this paper introduces a smooth function to approximate the check loss function so that the gradient based optimization methods could be employed for fitting quantile regression model. The properties of the smooth approximation are discussed. Two algorithms are proposed for minimizing the smoothed objective function. The first method directly applies gradient descent, resulting the gradient descent smooth quantile regression model; the second approach minimizes the smoothed objective function in the framework of functional gradient descent by changing the fitted model along the negative gradient direction in each iteration, which yields boosted smooth quantile regression algorithm. Extensive experiments on simulated data and real-world data show that, compared to alternative quantile regression models, the proposed smooth quantile regression algorithms can achieve higher prediction accuracy and are more efficient in removing noninformative predictors.
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Notes
For example, |x| is not smooth at x = 0, and its second order derivative at x = 0 could be understood as \(\infty\) in some sense.
A random variable X follows a double exponential distribution if its probability density function is \(f(x)=\frac{1}{2} e^{-|x|}. \)
Quantile regression forest was implemented based on the R package “quantregForest”, while all other algorithms in this paper were implemented using MATLAB, thus the computing time of QReg forest is not comparable to other algorithms. As such, we choose not to provide the training time of QReg forest.
IP-QReg and MM-QReg were implemented based on the MATLAB code downloaded from http://www.stat.psu.edu/∼dhunter/code/qrmatlab/. When the dimensionality is greater than the sample size, the software package gives error message. Thus, the performances of IP-QReg and MM-QReg are not provided.
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Acknowledgments
The short version of this paper [37] was published on the 9th Mexican International Conference on Artificial Intelligence (MICAI). The author gratefully acknowledges the anonymous reviewers of MICAI and International Journal of Machine Learning and Cybernetics for their constructive comments and would like to extend his gratitude to Prof. Grigori Sidorov for his excellent work in coordinating the preparation and the reviewing of the manuscript. This work was partially supported by a 2011 Summer Faculty Fellowship of Missouri State University.
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Zheng, S. Gradient descent algorithms for quantile regression with smooth approximation. Int. J. Mach. Learn. & Cyber. 2, 191–207 (2011). https://doi.org/10.1007/s13042-011-0031-2
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DOI: https://doi.org/10.1007/s13042-011-0031-2
Keywords
- Quantile regression
- Gradient descent
- Boosting
- Variable selection