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# 1.3. Kernel ridge regression

Kernel ridge regression (KRR) [M2012] combines Ridge regression and classification (linear least squares with l2-norm regularization) with the kernel trick. It thus learns a linear function in the space induced by the respective kernel and the data. For non-linear kernels, this corresponds to a non-linear function in the original space.

The form of the model learned by `KernelRidge` is identical to support vector regression (`SVR`). However, different loss functions are used: KRR uses squared error loss while support vector regression uses \(\epsilon\)-insensitive loss, both combined with l2 regularization. In contrast to `SVR`, fitting `KernelRidge` can be done in closed-form and is typically faster for medium-sized datasets. On the other hand, the learned model is non-sparse and thus slower than `SVR`, which learns a sparse model for \(\epsilon > 0\), at prediction-time.

The following figure compares `KernelRidge` and `SVR` on an artificial dataset, which consists of a sinusoidal target function and strong noise added to every fifth datapoint. The learned model of `KernelRidge` and `SVR` is plotted, where both complexity/regularization and bandwidth of the RBF kernel have been optimized using grid-search. The learned functions are very similar; however, fitting `KernelRidge` is approximately seven times faster than fitting `SVR` (both with grid-search). However, prediction of 100000 target values is more than three times faster with `SVR` since it has learned a sparse model using only approximately 1/3 of the 100 training datapoints as support vectors.

The next figure compares the time for fitting and prediction of `KernelRidge` and `SVR` for different sizes of the training set. Fitting `KernelRidge` is faster than `SVR` for medium-sized training sets (less than 1000 samples); however, for larger training sets `SVR` scales better. With regard to prediction time, `SVR` is faster than `KernelRidge` for all sizes of the training set because of the learned sparse solution. Note that the degree of sparsity and thus the prediction time depends on the parameters \(\epsilon\) and \(C\) of the `SVR`; \(\epsilon = 0\) would correspond to a dense model.

References:

[M2012]

“Machine Learning: A Probabilistic Perspective” Murphy, K. P. - chapter 14.4.3, pp. 492-493, The MIT Press, 2012