Alphabeta Math
AlgorithmAlgorithm: Literature-sourcedProof: Not applicablePipeline-generatedjudge pass (gpt-6-luna)
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Finite-horizon doubly robust off-policy value evaluation

Definition

Assume AC for the behavior-policy trajectory law. Input a finite-horizon logged dataset with n≥1 behavior trajectories, a target policy π, finite real functions Q^t(s,a) for 0≤t<H, and finite nonnegative action-weight multipliers ρ^t(s,a) for 0≤t<H. Hold the functions and multipliers fixed when taking expectation over the evaluation dataset; they may be conditioned on an independent training sample. Define V^H(s)=0 for every s, and for t=H−1,…,0 define

V^t(s)=∑a∈A(s)π(a∣s)Q^t(s,a).

For each logged trajectory i, set W^i,−1=1 and W^i,t=W^i,t−1ρ^t(Sti,Ati) for 0≤t<H. Compute

Di=V^0(s0)+∑t=0H−1γtW^i,t(Rt+1i+γV^t+1(St+1i)−Q^t(Sti,Ati)),

and return J^DR=n−1∑i=1nDi. When supplied as propensity ratios, the multipliers have the form ρ^t(s,a)=π(a∣s)/μ~t(a∣s), with positive denominators on target-positive actions. In the exact-ratio case they equal the target-to-behavior action-likelihood factors π(a∣s)/μ(a∣s); the procedure also permits finite fitted multipliers for the correct-value-model case. The fixed terminal value V^H=0 is part of the procedure.

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