## Section: New Results

### Comparison between multi-task and single-task oracle risks in kernel ridge regression

Participant : Matthieu Solnon [correspondent] .

In [35] we study multi-task kernel ridge regression and try to understand when the multi-task procedure performs better than the single-task one, in terms of averaged quadratic risk. In order to do so, we compare the risks of the estimators with perfect calibration, the oracle risk. We are able to give explicit settings, favorable to the multi-task procedure, where the multi-task oracle performs better than the single-task one. In situations where the multi-task procedure is conjectured to perform badly, we also show the oracle does so. We then complete our study with simulated examples, where we can compare both oracle risks in more natural situations. A consequence of our result is that the multi-task ridge estimator has a lower risk than any single-task estimator, in favorable situations.

Increasing the sample size is the most common way to improve the performance of statistical estimators. In some cases (see, for instance, the experiments of [56] on customer data analysis or those of [63] on molecule binding problems), having access to some new data may be impossible, often due to experimental limitations. One way to circumvent those constraints is to use datasets from several related (and, hopefully, “similar”) problems, as if it gave additional (in some sense) observations on the initial problem. The statistical methods using this heuristic are called “multi-task” techniques, as opposed to “single-task” techniques, where every problem is treated one at a time. In this paper, we study kernel ridge regression in a multi-task framework and try to understand when multi-task can improve over single-task.

The first trace of a multi-task estimator can be found in the work of [88] . In this article, Charles Stein showed that the usual maximum-likelihood estimator of the mean of a Gaussian vector (of dimension larger than 3, every dimension representing here a task) is not admissible—that is, there exists another estimator that has a lower risk for every parameter. He showed the existence of an estimator that uniformly attains a lower quadratic risk by shrinking the estimators along the different dimensions towards an arbitrary point. An explicit form of such an estimator was given by [64] , yielding the famous James-Stein estimator. This phenomenon, now known as the “Stein's paradox”, was widely studied in the following years and the behaviour of this estimator was confirmed by empirical studies, in particular the one from [55] . This first example clearly shows the goals of the multi-task procedure: an advantage is gained by borrowing information from different tasks (here, by shrinking the estimators along the different dimensions towards a common point), the improvement being scored by the global (averaged) squared risk. Therefore, this procedure does not guarantee individual gains on every task, but a global improvement on the sum of those task-wise risks.

We consider $p\ge 2$ different regression tasks, a framework we refer to as “multi-task” regression, and where the performance of the estimators is measured by the fixed-design quadratic risk.
Kernel ridge regression is a classical framework to work with and comes with a natural norm, which often has desirable properties (such as, for instance, links with regularity).
This norm is also a natural “similarity measure” between the regression functions.
[56] showed how to extend kernel ridge regression to a multi-task setting, by adding a regularization term that binds the regression functions along the different tasks together.
One of the main questions that is asked is to assert whether the multi-task estimator has a lower risk than any single-task estimator.
It was recently proved by [86] that a fully data-driven calibration of this procedure is possible, given some assumptions on the set of matrices used to regularize—which correspond to prior knowledge on the tasks.
Under those assumptions, the estimator is showed to verify an *oracle inequality*, that is, its risk matches (up to constants) the best possible one, the *oracle risk*.
Thus, it suffices to compare the oracle risks for the multi-task procedure and the single-task one to provide an answer to this question.

We study the oracle multi-task risk and compare it to the oracle single-task risk. We then find situations where the multi-task oracle is proved to have a lower risk than the single-task oracle. This allows us to better understand which situation favors the multi-task procedure and which does not. After having defined our model, we write down the risk of a general multi-task ridge estimator and see that it admits a convenient decomposition using two key elements: the mean of the tasks and the resulting variance. This decomposition allows us to optimize this risk and get a precise estimation of the oracle risk, in settings where the ridge estimator is known to be minimax optimal. We then explore several repartitions of the tasks that give the latter multi-task rates, study their single-task oracle risk and compare it to their respective multi-task rates. This allows us to discriminate several situations, depending whether the multi-task oracle either outperforms its single-task counterpart, underperforms it or whether both behave similarly. We also show that, in the cases favorable to the multi-task oracle detailed in the previous sections, the estimator proposed by [86] behaves accordingly and achieves a lower risk than the single-task oracle. We finally study settings where we can no longer explicitly study the oracle risk, by running simulations, and we show that the multi-task oracle continues to retain the same virtues and disadvantages as before.