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# What is the goal of a policy gradient?

## What is the goal of a policy gradient?

Policy Gradient The goal of reinforcement learning is to find an optimal behavior strategy for the agent to obtain optimal rewards. The policy gradient methods target at modeling and optimizing the policy directly. The policy is usually modeled with a parameterized function respect to θ, πθ(a | s).

## How do we add entropy to the loss to encourage exploration?

While training, we want to reduce the loss. In the beginning of training, almost all actions have same probability. After some training, some actions get higher probability (in the direction of getting more rewards), and entropy is reduced over time. However, I am confused, how adding entropy to loss will encourage exploration?

## When to omit θ in a policy gradient algorithm?

For simplicity, the parameter θ would be omitted for the policy πθ when the policy is present in the subscript of other functions; for example, dπ and Qπ should be dπθ and Qπθ if written in full.

## How are policy gradient algorithms used in reinforcement learning?

The goal of reinforcement learning is to find an optimal behavior strategy for the agent to obtain optimal rewards. The policy gradient methods target at modeling and optimizing the policy directly. The policy is usually modeled with a parameterized function respect to θ, πθ(a | s).

## How are policy gradient methods used in reinforcement learning?

Policy-Gradient methods are a subclass of Policy-Based methods that estimate an optimal policy’s weights through gradient ascent. Summary of approaches in Reinforcement Learning presented until know in this series. The classification is based on whether we want to model the value or the policy (source: https://torres.ai)

## How is the policy gradient method used in AI?

The policy gradient method will iteratively amend the policy network weights (with smooth updates) to make state-action pairs that resulted in positive return more likely, and make state-action pairs that resulted in negative return less likely.

## When do policy gradient algorithms have no bias?

Where refers to when both state and action distributions follow the policy (on policy). The policy gradient theorem lays the theoretical foundation for various policy gradient algorithms. This vanilla policy gradient update has no bias but high variance.

## When to use lil’log for policy gradient?

When k = 0: ρπ(s → s, k = 0) = 1. When k = 1, we scan through all possible actions and sum up the transition probabilities to the target state: ρπ(s → s ′, k = 1) = ∑aπθ(a | s)P(s ′ | s, a). Imagine that the goal is to go from state s to x after k+1 steps while following policy πθ.

## How are policy gradients defined in machine learning?

Like any Machine Learning setup, we define a set of parameters θ (e.g. the coefficients of a complex polynomial or the weights and biases of units in a neural network) to parametrize this policy — π_θ ​ (also written a π for brevity). If we represent the total reward for a given trajectory τ as r ( τ ), we arrive at the following definition.

## What is the relation between Q-learning and policy gradients methods?

Thus, policy gradient methods are on-policy methods. Q-Learning only makes sure to satisfy the Bellman-Equation. This equation has to hold true for all transitions. Therefore, Q-learning can also use experiences collected from previous policies and is off-policy.

## How is a policy gradient equivalent to a cross entropy loss?

The log expression of the policy gradient as shown below is equivalent to log expression of categorical cross-entropy loss. πθ (at, st) gives the action a to be taken to reach next state st+1 from the given state s at the time step t. Here is the explanation of that: This enables us to use the cross-entropy loss in the policy gradient algorithms.

What are Policy Gradient Methods? Policy gradient methods are a subclass of policy-based methods. It estimates the weights of an optimal policy through gradient ascent by maximizing expected cumulative reward which an agent gets after taking optimal action in a given state. Reinforcement learning is divided into two types of methods:

## Can a policy gradient beat a value based method?

In general, policy gradient methods have very often beaten value-based methods such as DQNs on modern tasks such as playing Atari games.