# The Learning Problem

In Machine Learning, an agent is said to be learning if it improves its performance P on task T based on experience/observations/data E. T must be fixed, P must be measurable, E must exist. See Learning agents.

## Types of Learning

### Active vs Passive Learning

An active learner interacts with the environment at training time, (e.g. by posing queries or performing experiments), while a passive learner only observes the information provided by the environment.

### Online vs Batch Learning

In online learning, the hypothesis has to be updated each time a new label is received.

### Reinforcement Learning

Each action in a state has an associated cost and a probability distribution of the next state.

Goal is to learn a policy (mapping from state to action) that minimizes the sum of expected current and future costs.

### Supervised Learning

Since learning involves an interaction between the learner and the environment, one can divide tasks according to the nature of that interaction.

Supervised learning describes a scenario in which the training examples contain significant information that is missing in the unseen “test examples”. In unsupervised learning, there is no distinction between the training and test data. The goal is to come up with some summary, or compressed version of that data.

#### Measure of success

A loss function helps measure our success. Given a set $$H$$ of hypothesis of models, and a domain $$Z$$, let $$l$$ be a function from $$H \times Z$$ to non-negative real numbers $$l: H \times Z \rightarrow \mathbb{R}_{+}$$.

The risk function is the expected loss of the hypothesis,

\begin{equation*} L_D(h) = E_{z \sim D}[l(h,z)] \end{equation*}

We are interested in finding a hypothesis $$h$$ that has a small risk, or expected loss, typically using Empirical Risk Minimization.

1. One common assumption is that the data in the data generation process is independently and identically distributed (IID), according to the distribution $$D$$.

Q: Given a large enough training set, do you expect the long term test error to be similar to the training error?

• If IID, then yes
• If not, there is likely dependencies, but under certain conditions, yes.
• If sampling mixes well, it will not take long for D’ to look like a steady set distribution.
• If dependencies are exploited, there is a possibility of attaining lower training and test error.

## Is Learning Feasible?

The target function $$f$$ that we want to learn is unknown. The performance of a hypothesis on the training set $$D$$ tells us nothing about the performance on the data outside of $$D$$.

As long as $$f$$ is unknown, knowing $$D$$ cannot exclude any patterns of $$f$$ outside of $$D$$, and the predictions of $$g$$ would be meaningless.

### Probabilistic View

If we accept a probabilistic answer, that is $$D$$ tells us something likely about $$f$$ outside of $$D$$, then learning is feasible, only with a small price.

Learning a hypothesis $$g$$ approximates the target function $$f$$ well, i.e. $$E_{out}(g) \approx 0$$. However, probabilistic analysis via Hoeffding’s Inequality gives $$E_{out}(g) \approx E_{in}(g)$$. Therefore, we still need to ensure $$E_{in}(g) \approx 0$$.