Models: Learning from Data

Okay, we've established that machine learning systems learn from data, specifically from the features (inputs) and sometimes labels (outputs) within that data. But what exactly is the result of this learning process? That result is what we call a model.

Think of a machine learning model as a specific representation of the patterns discovered in your training data. It's the artifact created by running a machine learning algorithm on your dataset. The goal is for this model to capture the underlying relationships between the input features and, if applicable, the output labels, so it can make useful predictions or decisions on new, unseen data.

What Does a Model Do?

At its core, a model takes new input data (features) for which you don't know the outcome and produces an output, which is typically a prediction or a decision.

• For Regression tasks (predicting continuous values, like house prices), the model might output a number. For example, given the square footage, number of bedrooms, and location (features) of a house, the model predicts its price (output).
• For Classification tasks (predicting discrete categories, like spam detection), the model might output a class label. Given the sender, subject line, and content (features) of an email, the model predicts whether it's "spam" or "not spam" (output).
• For Clustering tasks (grouping similar data points), the model effectively is the definition of the groups found in the data. Given a customer's purchase history (features), the model might assign them to a specific customer segment (output group).

Consider a simple analogy: Imagine trying to figure out the relationship between the hours you study ($x$) and the score you get on a test ($y$). You collect data from several tests (your training data). The machine learning algorithm is the method you use to analyze this data (maybe plotting points and drawing a line). The model is the specific line you end up drawing, perhaps represented by an equation like $y = 5x + 40$. This equation (the model) captures the pattern you observed and can now be used to predict your score ($y$) for a new amount of study time ($x$).

The learning algorithm processes training data to produce the trained model. This model can then make predictions on new input data.

Models are Learned, Not Explicitly Programmed

This is a fundamental difference from traditional programming highlighted in the previous chapter. Instead of writing explicit rules (if email contains 'free money' then mark as spam), we provide data and let an algorithm learn the rules or patterns that constitute the model. The complexity of these patterns can range from a simple linear equation, as in our study example, to incredibly complex structures capable of recognizing images or translating languages.

Model vs. Algorithm

It's helpful to distinguish between the algorithm and the model:

• Algorithm: The procedure or set of rules used to learn from data. Examples include Linear Regression, K-Nearest Neighbors, or K-Means (which we will discuss later). It's the recipe for learning.
• Model: The specific output or result of applying a learning algorithm to a particular dataset. It's the learned representation, like the equation $y = 5x + 40$ derived from the study data using the Linear Regression algorithm. It's the cake baked using the recipe.

You use an algorithm on your data to train a model. This trained model is then saved and used for making future predictions. In the upcoming sections, we'll look closer at how models actually learn by adjusting their internal settings, known as parameters, and how we configure the learning process using hyperparameters.