Almost Sure Policy Convergence for Stochastic Bandits

Poster E: Wednesday -- 11:00 - 12:30

Liam Schramm, Ronald Ortner

Keywords: Bandit theory, high probability regret

A stochastic bandit algorithm can attain optimal regret yet still behave erratically, playing badly suboptimal actions in long bursts arbitrarily late in training. We argue that a satisfying guarantee should control not just cumulative regret but the \emph{convergence} of the policy: the policy's expected suboptimality $\Delta_t$ should be bounded by a known decreasing function at every time step, with high probability. The suboptimality of the \emph{realized action} cannot converge in this sense without sacrificing exploration, but the suboptimality of the \emph{policy}---the expected gap before the action is sampled---can. We formalize this as \emph{almost sure policy convergence} and show that it implies bounds on expected regret, high-probability regret, and a policy-based version of satisficing regret that is constant in the horizon. We give an algorithm that achieves almost sure policy convergence at a near-optimal rate, yielding $O(\sum_i \log^2 T/\Delta_i)$ classical regret and horizon-free constant satisficing regret simultaneously. Matching lower bounds show the convergence rate is tight to within a logarithmic factor. Finally, we prove that algorithms whose per-arm probabilities are bounded away from zero, such as UCB and successive elimination, cannot achieve this combined guarantee.