Contents

- 1 What is the target Q-value in dqns?
- 2 How is target update delayed in reinforcement learning?
- 3 Is there such a thing as two Q-targets?
- 4 How are q values determined in a deep Q Network?
- 5 What happens when you take the maximum Q value?
- 6 How are fixed q targets used in DeepMind?
- 7 When does DQN use a standard deep network?
- 8 How often does DQN update the target model?

## What is the target Q-value in dqns?

In a DQN, which uses off-policy learning, they represent a refined estimate for the expected future reward from taking an action a in state s, and from that point on following a target policy. The target policy in Q learning is based on always taking the maximising action in each state, according to current estimates of value.

## How is target update delayed in reinforcement learning?

This delayed update of the target predictions is done for numerical stability in DQN – conceptually it is an estimate for the same action values that you are learning. This target value can change every time you use any specific memory from experience replay.

**How are q values used to train the Q Network?**

The idea is that using the target network’s Q values to train the main Q-network will improve the stability of the training. Later, when we present the code of the training loop, we will enter in more detail how to code the initialization and use of this target network.

**Which is the second post in the Deep Q Network series?**

Deep Q-Network (DQN)-II. Experience Replay and Target Networks | by Jordi TORRES.AI | Towards Data Science This is the second post devoted to Deep Q-Network (DQN), in the “Deep Reinforcement Learning Explained” series, in which we will analyse some challenges that appear when we apply Deep Learning to Reinforcement Learning.

### Is there such a thing as two Q-targets?

Instead of using one Neural Network, it uses two. Yes, you heard that right! (like one wasn’t enough already). One as the main Deep Q Network and a second one (called Target Network) to update exclusively and periodically the weights of the target. This technique is called Fixed Q-Targets.

### How are q values determined in a deep Q Network?

Deep Q Networks take as input the state of the environment and output a Q value for each possible action. The maximum Q value determines, which action the agent will perform.

**Why do we copy parameters from DQN to Tau?**

Using a separate network with a fixed parameter (let’s call it w-) for estimating the TD target. At every Tau step, we copy the parameters from our DQN network to update the target network. Thanks to this procedure, we’ll have more stable learning because the target function stays fixed for a while.

**How are next state q values calculated in ddpg?**

However, in DDPG, the next-state Q values are calculated with the target value network and target policy network. Then, we minimize the mean-squared loss between the updated Q value and the original Q value: * Note that the original Q value is calculated with the value network, not the target value network.

## What happens when you take the maximum Q value?

Therefore, taking the maximum q value (which is noisy) as the best action to take can lead to false positives. If non-optimal actions are regularly given a higher Q value than the optimal best action, the learning will be complicated.

## How are fixed q targets used in DeepMind?

Instead, we can use the idea of fixed Q-targets introduced by DeepMind: Using a separate network with a fixed parameter (let’s call it w-) for estimating the TD target. At every Tau step, we copy the parameters from our DQN network to update the target network.

**Is the target network the same as the Q Network?**

The Target network is identical to the Q network. The DQN gets trained over multiple time steps over many episodes. It goes through a sequence of operations in each time step: Now let’s zoom in on this first phase.

**How is the Q Network used in reinforcement learning?**

The Q network takes the current state and action from each data sample and predicts the Q value for that particular action. This is the ‘Predicted Q Value’. The Target network takes the next state from each data sample and predicts the best Q value out of all actions that can be taken from that state. This is the ‘Target Q Value’.

### When does DQN use a standard deep network?

This happens basically all the time with DQN when using a standard deep network (bunch of layers of the same size fully connected). The effect you typically see with this is referred to as “catastrophic forgetting” and it can be quite spectacular.

### How often does DQN update the target model?

The target model is supposed to have the same function as the policy model, but the DQN algorithm purposely separates them and updates it once in a while to stabilize training. It can be done once every certain number of steps, or in this case they seem to do it every episode.

**How does the ddqn help in the estimation?**

By decoupling the estimation, intuitively our DDQN can learn which states are (or are not) valuable without having to learn the effect of each action at each state (since it’s also calculating V (s) ). We’re able to calculate V (s).

**What’s the difference between a DQN and a ddqn?**

DQN tend to be overoptimistic. It will over-appreciate being in this state although this only happened due to the statistical error (Double DQN solves it)