# Tag Info

### What is the relation between online (or offline) learning and on-policy (or off-policy) algorithms?

The concepts of on-policy vs off-policy and online vs offline are separate, but do interact to make certain combinations more feasible. When looking at this, it is worth also considering the ...
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### Why do we need importance sampling?

Importance sampling is typically used when the distribution of interest is difficult to sample from - e.g. it could be computationally expensive to draw samples from the distribution - or when the ...
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### How can the importance sampling ratio be different than zero when the target policy is deterministic?

You're correct, when the target policy $\pi$ is deterministic, the importance sampling ratio will be $\geq 1$ along the trajectory where the behaviour policy $b$ happened to have taken the same ...
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### Why is the n-step tree backup algorithm an off-policy algorithm?

As for your first two questions: there is indeed a behaviour and a target policy, which can be different. In the example image of the $3$-step tree-backup update in the beginning of the section you ...
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### Why does off-policy learning outperform on-policy learning?

This post contains many answers that describe the difference between on-policy vs. off-policy. Your book may be referring to how the current (DQN-based) state-of-the-art (SOTA) algorithms, such as Ape-...
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### Which policy do I need to use in updating Q function?

I am going to stick with Q learning here to keep things simple. Most value-based reinforcement learning used for optimal control will have some statement similar to: Choose $a$ from $s$ using policy ...
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### Why is DDPG an off-policy RL algorithm?

DDPG is an off-policy algorithm simply because of the objective taking expectation with respect to some other distribution that we are not learning about, i.e. the deterministic policy gradient can be ...
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### What is meant by "generate the data" in describing the difference between on-policy and off-policy?

In the book, the phrase "generate the data" refers to the data from observations about states, actions, next states and rewards, that then get used to make value estimate updates. In both ...
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### Is Expected SARSA an off-policy or on-policy algorithm?

Expected SARSA can be used either on-policy or off-policy. The policy that you use in the update step determines which it is. If the update step uses a different weighting for action choices than the ...
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### Are model-free and off-policy algorithms the same?

In respect of RL, is model-free and off-policy the same thing, just different terminology? No, they are entirely different terms, with the only thing they have in common is that they are both ways ...
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### Understanding the n-step off-policy SARSA update

Multiplying the entire update by $\rho$ has the desirable property that experience affects $Q$ less when the behavior policy is unrelated to the target policy. In the extreme, if the trajectory taken ...
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### Can I add expert data to the replay buffer used by the DDPG algorithm in order to make it converge faster?

What I want to know is whether I can add expert data to the replay buffer, given that DDPG is an off-policy algorithm? You certainly can, that is indeed one of the advantages of off-policy learning ...
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### Could we update the policy network with previous trajectories using supervised learning?

You cannot really do that because you have no way of knowing how good the action really is to make reasonable labels for supervised learning (that's the whole point why we need reinforcement learning)....
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### What is the intuition behind importance sampling for off-policy value evaluation?

In the application of importance sampling to RL, is the expectation of the function $f$ equivalent to the value of the trajectories, which is represented by the trajectories $x$? I believe what you ...
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### What is the difference between on-policy and off-policy for continuous environments?

First, some preliminary questions: in this case, what is the optimal policy? It is the policy that maximises return from any given time step $G_t$. You need to be careful with your definition of ...
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### What is the intuition behind importance sampling for off-policy value evaluation?

Let's fix some notation: we're collecting data from behavior policy $\pi_0$ and we want to evaluate a policy $\pi$. Of course, if we had plenty of data from policy $\pi$ that would be the best way to ...
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### How does this TD(0) off-policy value update formula work?

This would mean we decrease the value of this state. Yes. This update that reduces the estimate is correct because it adjusts for the inevitable over-estimate of value when the exploration policy ...
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1 vote

### When learning off-policy with multi-step returns, why do we use the current behaviour policy in importance sampling?

According to my understanding, you don't use just the current behavior policy for sampling. The importance sampling ratio is calculated as the product of the probability ratios for both the target and ...
1 vote

### Learning only using off-policy samples

What you're describing is off-policy learning. A classic example is $Q$-learning, where you follow some policy $\pi$ whilst learning about the greedy policy. If you're interested in actor-critic ...
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1 vote

### How do I know that the DQN has learnt an appropriate Q function?

DQN is famous for doing over-approximation on Q function. However, having over approximated Q does not imply that it does not perform well in the environment. (unless it looks ridiculously high) From ...
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