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What's the difference between model-free and model-based reinforcement learning? In Reinforcement Learning, the terms "model-based" and "model-free" do not refer to the use of a neural network or other statistical learning model to predict values, or even to predict next state (although the latter may be used as part of a model-based algorithm and be called ...


28

Model-based reinforcement learning has an agent try to understand the world and create a model to represent it. Here the model is trying to capture 2 functions, the transition function from states $T$ and the reward function $R$. From this model, the agent has a reference and can plan accordingly. However, it is not necessary to learn a model, and the ...


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In reinforcement learning (RL), there is an agent which interacts with an environment (in time steps). At each time step, the agent decides and executes an action, $a$, on an environment, and the environment responds to the agent by moving from the current state (of the environment), $s$, to the next state (of the environment), $s'$, and by emitting a scalar ...


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Tabular Q-Learning does not explicitly create a model of the transition function. It does not generate any output that you can afterwards use as a function to predict what the next state s' will be given a current state s and an action a (that's what a transition function would allow you to do). So no, Q-learning is still model-free. By the way, model-based ...


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Although there are several good answers, I want to add this paragraph from Reinforcement Learning: An Introduction, page 303, for a more psychological view on the difference. The distinction between model-free and model-based reinforcement learning algorithms corresponds to the distinction psychologists make between habitual and goal-directed control ...


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The main idea is that you can estimate $V^\pi(s)$, the value of a state $s$ under a given policy $\pi$, even if you don't have a model of the environment, by visiting that state $s$ and following the policy $\pi$ after that state. If you repeat this process many times, you'll get many samples of trajectories starting at $s$ with some total return associated ...


4

Your two suggestions are not mutually exclusive. If you go by this process, you'll have to do a "Cartesian product" of a bunch of different RL categorizations which would get out of hand. I recommend, if you can, to describe some sort of "RL taxonomy" instead. By this I mean describing different RL characterizations without assuming they'...


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Yes, there are algorithms that try to predict the next state. Usually this will be a model based algorithm -- this is where the agent tries to make use of a model of the environment to help it learn. I'm not sure on the best resource to learn about this but my go-to recommendation is always the Sutton and Barto book. This paper introduces PlanGAN; the idea ...


4

In reinforcement learning (RL), there are some agents that need to know the state transition probabilities, and other agents that do not need to know. In addition, some agents may need to be able to sample the results of taking an action somehow, but do not strictly need to have access to the probability matrix. This might be the case if the agent is allowed ...


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In Model Based Reinforcement learning, state and state-action values for all states can be calculated based on the bellman equations. The equations are taken from Andrew Ng's Algorithms for Inverse Reinforcement Learning $$V^{\pi}(s) = R(s) + \gamma \sum_{s'}P(s'|s,a)V^{\pi}(s') \\ Q^{\pi}(s,a) = R(s) + \gamma \sum_{s'}P(s'|s,a)V^{\pi}(s')$$ In this setting, ...


3

Model-Free RL In Model-Free RL, the agent does not have access to a model of the environment. By environment I mean a function which predicts state transition and rewards. As of the time of writing, model-free methods are more popular and have been researched extensively. Model-Based RL In Model-Based RL, the agent has access to a model of the environment. ...


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In model-based learning, the learning agent utilizes a model that was previously learned to accomplish a task, whereas model-free RL doesn't use the environment to learn but simply relies on trial and error experience for action selection. The similarity is that in both methods the learning agent is trying to maximize reward from its actions. Q-Learning is a ...


2

According to OpenAI – Kinds of RL Algorithms, algorithms which use a model of the environment, i.e. a function which predicts state transitions and rewards, are called model-based methods, and those that don’t are called model-free. This model can either have been given the agent or learned by the agent. Using a model allows the agent to plan by thinking ...


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Everything you say in your post is correct, apart from the wrong assumption that policy iteration is model-free. PI is a model-based algorithm because of the reasons you're mentioning. See my answer to the question What's the difference between model-free and model-based reinforcement learning?.


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Minimax is a planning algorithm, and all planning algorithms need access to a model of the environment in order to look ahead or simulate possible future states and results. Technically this does not need to be 100% accurate or complete. It could even be a learned model. However, in the case of applying minimax to classic two player games, such as chess or ...


2

In this Medium article I found [1] it is quite well explained what is behind the better model efficiency in model based RL in comparison to model free one. Main difference between those two is like you said, that the model helps in finding the correct path more efficiently because of the existence of the model. Maybe you cannot find new samples (point 1) ...


2

Depending on your needs and the size of the project, you might be better off making a custom set of environments. If you'd rather not do that, though, you should take a look at OpenAI's CoinRun environment. A high-level description can be found in their blog post. The "RandomMazes" version of this environment might be useful to you. And if you want to make ...


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Is the main difference between these two problems, and hence why one is regression and the other is kernel density estimation, because with the reward we are mainly concerned with the expected reward (hence regression) whereas with the state transitioning, we want to be able to simulate this so we need the estimated density? Yes. An expected reward ...


2

Yes, the concept of dreaming or imagining has already been explored in reinforcement learning. For example, have a look at Metacontrol for Adaptive Imagination-Based Optimization (2017) by Jessica B. Hamrick et al., which is a paper that I gave a talk/presentation on 1-2 years ago (though I don't remember well the details anymore). There is also a blog post ...


2

However, if we can model the environment, why should we want to employ a model-free algorithm? Depends what you mean by "model the environment". There are two kinds of model: Distribution model, which provides full access to a function like $p(r,s'|s,a)$, the probability of observing reward $r$ and next state $s'$ given starting state $s$ and ...


2

For every visit MC you create a list for each state. Every time you enter a state you calculate the returns for the episode and append these returns to a list. Once you have done this for all the episode you want to average over you simply calculate the value of a state to be the average of this list of returns for the state. First visit MC is almost the ...


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Check out Imagination-Augmented Agents paper - seems like it does what you are talking about. The agent itself is the standard A3C that you are familiar with. The novelty is the "imagination" environment model which is trained to predict the behavior of the environment.


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I think that your description is roughly correct, but I wouldn't call a "sampling model" a "model" because it doesn't necessarily model something, unless, for example, you are first learning in simulation to later be able to act in the real-world or environment (in this sense, the simulation would be a model of the real environment, but ...


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I think there is an implicit notion of it in dynamic programming; say, if you have to make some sort of search over a subset of a state space and you are deciding whether to use BFS, breath first search, or DFS, depth first search, you are at least implicitly thinking on the best way to explore/exploit the state space. As for model based RL, yes. There is ...


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At a basic level, these kinds of low-dimensional plots where you look at one or two variables at a time can help to give you a sense of what types of relationships you might expect to see, such as linear, non-linear, or periodic relationships, which can steer you toward an appropriate family of models. You wouldn't want to use a linear model to predict data ...


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If you already have some transition tuples then you can train a model to predict environment dynamics using these. However, you should be careful that your pre-gathered data is diverse enough to 'cover' enough of the state/action space so that your model remains accurate. For instance, when you start training your agent it will likely start to see more of ...


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This is just a case of supervised learning. You are trying to predict $s_{t+1}$ given $s_t$ and $a_t$, so the answer to your question depends on how complex your state dynamics are. For example, if the state space is really complex, e.g. if your state space is an image and you want to predict the next image given the current image and an action, then linear ...


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If I understand your question correctly, the significance of this is due to the fact that $s'$ is random. In the RHS of the equation it is assumed that $V(\cdot)$ is known for each state, but the quantity is measuring the expected value of the next state given the current state and action.


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I will give one perspective on this from the domain of robotics. You are right that most RL agents are trained in simulation particularly for research papers, because it allows researchers to in theory benchmark their approaches in a common environment. Many of the environments exist strictly as a test bed for new algorithms and are not even physically ...


1

The original question about both the estimation of the transition model, often denoted as $T$, and the reward function, sometimes denoted as $R$, arose because I was thinking about the probability distribution often denoted as $$\color{red}{p}\left(s^{\prime}, r \mid s, a\right) \doteq \operatorname{Pr}\left\{S_{t}=s^{\prime}, R_{t}=r \mid S_{t-1}=s, A_{t-1}=...


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