In a previous post I talked about ad serving, and why regression based approaches still dominate even though other approaches to cost-sensitive multiclass classification (CSMC) have lower regret bounds. In my view, it comes down to practical issues, and an important practical issue in ad serving is that the set of actions (ads) that are allowed for a given decision instance (ad serving request) can be volatile. Furthermore in many cases there is no reason to believe the pattern of constraints is statistically stable between training sets and test sets, e.g., due to advertisers experimenting with budgets. Therefore I feel the constraints are best modeled adversarially, a situation I call minimax constrained CSMC.

I'll repeat the setting for minimax constrained CSMC. There is a distribution $D = D_x \times D_{\tilde c|x}$, where $\tilde c: K \to \mathbb{R}$ takes values in the regular reals $\mathbb{R}$. Then, an adversary comes in and manufactures a cost vector $c$ in the extended reals $\mathbf{R}$ by setting some of the components to $\infty$; these choices are revealed via $\omega$ prior to a decision being elicited. In this case the regret of a particular classifier $h: X \times \mathcal{P} (K) \to K$ is given by \[ \nu (h) = E_{x \sim D_x} \left[ \max_{\omega \in \mathcal{P} (K)} \left\{ E_{\tilde c \sim D_{\tilde c|x}} \left[ c (h (x, \omega)) \right] - \min_{k \in K}\; E_{\tilde c \sim D_{\tilde c|x}} \left[ c (k) \right] \right\} \right]. \] This contrasts with average constrained CSMC, where the distribution of constraints ($\omega$) is stable from training to test data. For average constrained CSMC, tree based reductions work when modified to have disallowed options forfeit their tournaments. This doesn't work for minimax constrained CSMC, however, as the following simple counterexample shows. Suppose $X = \emptyset$, $K = \{1, 2, 3\}$, and $\tilde c$ is deterministic and such that $\tilde c (1) < \tilde c (3) < \tilde c (2)$, and suppose the tree first pairs $\{1, 2\}$ while giving 3 a pass, and then pairs $\{1, 3\}$. Suppose the classifier used at each tree node is $1_{f (a) > f (b)}$ for some function $f: K \to \mathbb{R}$. If the training is done only with data where $\omega = \emptyset$, the regret on the training data can be brought to zero if $f (1) = 1$, $f (3) = 3$, and $f (2) = 2$. However when $\omega = \{1\}$ at test time there will be regret.

What's going on here? The situation is similar to a ranking reduction to classification, where $f$ induces a linear ordering over the elements. In that case the classification error averaged over input pairs provides a bound on the AUC error averaged over input sets. Of course, AUC is too coarse an objective function since it is only sensitive to ordering errors and not magnitudes. However this does suggest that more pairs of elements need to be compared during training other than the $(|K| - 1)$ comparisons done during one pass of the filter tree. If every pair must be compared during training, then perhaps $|K|/2$ passes over the filter tree are required.

Therefore consider a sequence of average constrained CSMC classifiers $\Psi_n$ indexed by $n \in [1, |K|]$. These induce a sequence of $\{ \tilde \omega_n | n \in [0, |K|] \}$ defined via \[

\begin{aligned}

\tilde \omega_0 &= \emptyset, \\

\tilde \omega_n &= \tilde \omega_{n-1} \cup \{ \Psi_n (x, \tilde \omega_{n-1}) \}.

\end{aligned}

\] Essentially this is a sequence of average constrained CSMC classifiers that are determining the best action, the next best action, and so on (in the same fashion as reduction from cost-sensitive best m to cost-sensitive multiclass). Next consider the index \[

\eta (x, \omega) = \min \{ n \in [1, |K|] | \Psi_n (x, \tilde \omega_{n-1}) \not \in \omega \}. \] If $\omega \neq K$, this index always exists. I'll construct a classifier when $\omega \neq K$ via \[ h (x, \omega) = \Psi_{\eta (x, \omega)} (x, \tilde \omega_{\eta (x, \omega) -1}).

\] (When $\omega = K$, the regret is always zero whatever choice the classifier makes, so I'll just ignore that case going forward). The regret for a particular $(x, \omega)$ is given by \[

\begin{aligned}

\nu (x, \omega) &= E_{\tilde c \sim D_{\tilde c|x}} \left[ c (h (x, \omega)) \right] - \min_{k \in K}\; E_{\tilde c \sim D_{\tilde c|x}} \left[ c (k) \right] \\

&= E_{\tilde c \sim D_{\tilde c|x}} \left[ c (h (x, \omega)) \right] - \min_{k \in K \setminus \tilde \omega_{\eta (x, \omega) - 1}} E_{\tilde c \sim D_{\tilde c|x}} \left[ c (k) \right] \\

&= E_{\tilde c \sim D_{\tilde c|x}} \left[ c \left(\Psi_{\eta (x, \omega)} (x, \tilde \omega_{\eta (x, \omega) -1})\right) \right] - \min_{k \in K \setminus \tilde \omega_{\eta (x, \omega) - 1}} E_{\tilde c \sim D_{\tilde c|x}} \left[ c (k) \right], \\

\end{aligned}

\] where the second line follows from $\tilde \omega_{\eta (x, \omega) - 1} \subseteq \omega$, and the third line from the definition of $h$. Now the last line is the per-instance regret of the $\eta (x, \omega)^{\textrm{th}}$ average constrained CSMC classifier trained on the distribution induced by the first $(\eta (x, \omega) - 1)$ classifiers. Thus \[

\max_{\omega \in \mathcal{P} (K)} \nu (x, \omega) = \max_n \left\{ E_{\tilde c \sim D_{\tilde c|x}} \left[ c (\Psi_n (x, \tilde \omega_n)) \right] - \min_{k \in K \setminus \tilde \omega_{n - 1}} E_{\tilde c \sim D_{\tilde c|x}} \left[ c (k) \right] \right\}.

\] This suggests a procedure where $|K|$ forfeit filter tree passes are done per instance; while this seems like a factor of 2 too many, note forfeitures do not generate classification instances which eliminates half of the comparisons. The minimax constrained CSMC regret would be \[

\nu (h) \leq (|K| - 1) E_{x \sim D_x} \left[ \max_n\; q_n (x, \tilde \omega_{n-1}) \right]

\] where $q_n (x, \tilde \omega_{n-1})$ is the average per-node importance-weighted regret of the $n^{\textrm{th}}$ forfeit filter tree trained on the distribution induced by the first $(n-1)$ forfeit filter trees.

At first blush this seems too unwieldy to use in practice, but two modifications might make it practical. The first is to reuse the same tree for every $\Psi_n$ instead of keeping $n$ separate trees; the regret bound still holds, although the proper training procedure is not immediately obvious to me. The second is to consider the case where the number of constraints are bounded, i.e., $|\omega| \leq z \ll |K|$, such that training and testing costs are only increased by $z$. This seems reasonable in practice.

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