Direct Optimization of Hyper-Parameter

In the previous post (https://statcompute.wordpress.com/2019/02/03/sobol-sequence-vs-uniform-random-in-hyper-parameter-optimization), it is shown how to identify the optimal hyper-parameter in a General Regression Neural Network by using the Sobol sequence and the uniform random generator respectively through the N-fold cross validation. While the Sobol sequence yields a slightly better performance, outcomes from both approaches are very similar, as shown below based upon five trials with 20 samples in each. Both approaches can be generalized from one-dimensional to multi-dimensional domains, e.g. boosting or deep learning.

net <- grnn.fit(scale(Boston[, -14]), Boston[, 14], sigma = 1)
                        
sb_out <- Reduce(rbind, Map(function(x) grnn.cv(net, gen_sobol(0.1, 1.0, 20, x), 4, 2019), seq(1, 5)))

uf_out <- Reduce(rbind, Map(function(x) grnn.cv(net, gen_unifm(0.1, 1.0, 20, x), 4, 2019), seq(1, 5)))

Map(function(x) x[x$R2 == max(x$R2), ], list(sobol = sb_out, uniform = uf_out))
# $sobol
#  sigma        R2
# 0.5568 0.8019342
# $uniform
#  sigma        R2
# 0.5608 0.8019327

Other than the random search, another way to locate the optimal hyper-parameter is applying general optimization routines, As shown in the demonstration below, we first need to define an objective function, e.g. grnn.optim(), to maximize the Cross-Validation R^2. In addition, depending on the optimization algorithm, upper and lower bounds of the parameter to be optimized should also be provided. Three optimization algorithms are employed in the example, including unconstrained non-linear optimization, particle swarm optimization, and Nelder–Mead simplex optimization, with all showing comparable outcomes to ones achieved by the random search.

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