In our exploration of Deep Q-Networks (DQN), we saw how combining Q-learning with deep neural networks and techniques like experience replay and target networks allows agents to learn effectively in high-dimensional state spaces. However, the standard Q-learning update used in DQN can suffer from a significant issue: the overestimation of Q-values.
Recall the target value (Yt) calculation in standard DQN for a transition (St,At,Rt+1,St+1):
YtDQN=Rt+1+γa′maxQ(St+1,a′;θt−)Here, θt− represents the parameters of the target network. The update for the online network parameters θt aims to minimize the loss between Q(St,At;θt) and this target YtDQN.
The problem lies in the maxa′ operation within the target calculation. The Q-values estimated by the target network (Q(St+1,a′;θt−)) are inherently noisy approximations of the true action values. When we take the maximum over these noisy estimates, we are more likely to pick an overestimated value than an underestimated one. Imagine several actions whose true values are similar; due to noise, some estimates will be higher than others. The max
operator will consistently select these higher, potentially overestimated, values. This systematic positive bias can propagate through the learning process, leading to overly optimistic value estimates, instability during training, and potentially suboptimal policies.
Double Deep Q-Networks (DDQN), proposed by Hado van Hasselt, Guilià Ghesu, Matej Horgan, Maurice Wiering, and David Silver, directly address this overestimation problem by decoupling the selection of the best action from the evaluation of that action's value when calculating the target.
Instead of using the target network θt− for both selecting the maximizing action and evaluating its Q-value, DDQN uses the online network θt to select the best action for the next state St+1, and then uses the target network θt− to evaluate the Q-value of that specific chosen action.
The target value calculation in DDQN becomes:
YtDDQN=Rt+1+γQ(St+1,arga′maxQ(St+1,a′;θt);θt−)Let's break this down:
Comparison of target value calculation flow in standard DQN and Double DQN (DDQN). DDQN uses the online network for action selection (argmax) and the target network for evaluating the selected action's value.
The online network (θt) and the target network (θt−) are different sets of parameters (the target network is usually a periodically updated copy of the online network). While both networks might have noise and potential overestimations for certain actions, it's less likely that both networks will overestimate the value of the same suboptimal action simultaneously.
If the online network selects an action a∗ because its estimate Q(St+1,a∗;θt) is currently overestimated, the target network's estimate Q(St+1,a∗;θt−) for that same action might be closer to the true value (or at least, less overestimated). By using the target network's value for the online network's chosen action, DDQN reduces the chance of propagating the maximum possible overestimation into the target value Yt. This leads to more conservative and accurate value estimates.
Implementing DDQN is remarkably straightforward if you already have a DQN implementation. The only change required is modifying how the target value Yt is calculated during the training loop. You still need experience replay and separate online and target networks.
The primary benefits observed with DDQN are:
Because the modification is simple and the benefits are significant and consistent across many domains, DDQN is considered a standard improvement over the original DQN algorithm and is often used as a default choice or baseline. It represents an important step in refining value-based deep reinforcement learning methods.
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