# Questions tagged [model-free-methods]

For questions about model-free reinforcement learning methods (or algorithms). An example of a model-free algorithm is Q-learning, which does not use the transition function (i.e. the model) of the environment (or Markov decision process).

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### How to prove importance sampling ratio is uncorrelated with action-value (or state-value) estimate?

In Sutton & Barto (2nd edition), the following is mentioned on page 150 (p. 172 of the pdf), section 7.4: the importance sampling ratio has expected value one (Section 5.9) and is uncorrelated ...
38 views

### Why don't we bootstrap terminal state in n-step temporal difference prediction update equation?

In the algorithm below, when $\tau + n \geq T$, shouldn't the algorithm bootstrap with the value of the next state? For instance, when $T=5, \tau=3, \& \; n=2$, we don't bootstrap the sample ...
16 views

### Why does one-step TD strengthen only the last action of the sequence of actions that led to the high reward, while n-step TD the last n actions?

In the caption of figure 7.4 (p. 147) of Sutton & Barto's book (2nd edition), it's written The one-step method strengthens only the last action of the sequence of actions that led to the high ...
37 views

### How does n-step Temporal Difference remove the notion of time-step?

How does n-step TD removes the notion of time-step as referenced in Sutton and Barto (2nd edition, Page 163) below? Another way of looking at the benefits of n-step methods is that they free you from ...
32 views

### In deep reinforcement learning, what is this model with state as input and value as output?

I was looking at this implementation for creating an agent for playing Tetris using DeepRL. This model uses "a state based on the statistics of the board after a potential action. All predictions ...
53 views

### If we can model the environment, wouldn't be meaningless to use a model-free algorithm?

I am trying to understand the concept of model-free and model-based approaches. As far as I understand, having a model of the environment does not mean that an RL agent has to be model-based. It is ...
70 views

### What kind of reinforcement learning method does AlphaGo Deepmind use to beat the best human Go player?

In reinforcement learning, there are model-based versus model-free methods. Within model-based ones, there are policy-based and value-based methods. AlphaGo Deepmind RL model has beaten the best Go ...
130 views

### Why does Monte Carlo policy evaluation relies on action-value function rather than state-value function?

Here is David Silver's lecture on that. Look at 9:30 to 10:30. He says that, since it is model-free learning, the environment's dynamics are unknown, so the action-value function $Q$ is used. But ...
103 views

### Why are state-values alone not sufficient in determining a policy (without a model)?

"If a model is not available, then it is particularly useful to estimate action values (the values of state-action pairs) rather than state values. With a model, state values alone are sufficient ...
92 views

### Into which subcategories can reinforcement learning be divided?

In the course of a scientific work, I will discuss the different types of reinforcement learning. However, I have difficulties to find these different types. So, into which subcategories can ...
81 views

### How can the policy iteration algorithm be model-free if it uses the transition probabilities?

I'm actually trying to understand the policy iteration in the context of RL. I read an article presenting it and, at some point, a pseudo-code of the algorithm is given : What I can't understand is ...
106 views

### How does policy evaluation work for continuous state space model-free approaches?

How does policy evaluation work for continuous state space model-free approaches? Theoretically, a model-based approach for the discrete state and action space can be computed via dynamic programming ...
181 views

### Is the minimax algorithm model-based?

Trying to get my head around model-free and model-based algorithms in RL. In my research, I've seen the search trees created via the minimax algorithm. I presume these trees can only be created with a ...
737 views

### 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? If not, what are the differences? I've read that the policy can be thought of as 'the brain', or decision ...
677 views

### Why are model-based methods more sample efficient than model-free methods?

Why do model-based methods use fewer samples than model-free methods? Here, I'm specifically referring to model-based methods in which we have to learn a policy and model. I can only think of two ...
338 views

### What is the relation between Monte Carlo and model-free algorithms?

Monte Carlo (MC) methods are methods that use some form of randomness or sampling. For example, we can use an MC method to approximate the area of a circle inside a square: we generate random 2D ...
436 views

### How can we estimate the transition model and reward function?

In reinforcement learning (RL), there are model-based and model-free algorithms. In short, model-based algorithms use a transition model (e.g. a probability distribution) and the reward function, even ...
In value iteration, we have a model of the environment's dynamics, i.e $p(s', r \mid s, a)$, which we use to update an estimate of the value function. In the case of temporal-difference and Monte ...