In machine learning we can find three main different branches where we can classify the algorithms:

• Supervised learning.
• Unsupervised learning.
• Reinforcement learning.

Supervised learning

In supervised algorithms you know the input and the output that you need from your model. You do not know how the output is achieved from the input data or how are the inner relations among you data, but definitely know the output data.

As an example, we can take a magazine publication that it has the subscription data of a determinate number of customers or old customers, let’s say 100.000 customers. The company in charge of the magazine knows that half of these customers (50.000) have cancelled their subscriptions and the other half (50.000) are still subscribed, and they want a model to predict what customers will cancel their subscriptions.

We know the input: customers subscription data, and the output: cancelled or not.

We can then build our training data set with 90.000 customers data. Half of them cancelled and half of them still active. We will train our system with this training set. And after that we will try to predict the result for the other 10.000 we left outside the training data to check the accuracy of our model.

Unsupervised learning

In unsupervised learning algorithms you do not know what is the output of your model, you maybe know there is some kind of relation or correlation in your data but, maybe, the data is too complex to guess.

In this kind of algorithms, you normalize your data in ways that it can be compared and you wait for the model to find some of these relationships. One of the special characteristics of these models is that, while the model can suggest different ways to categorize or order your data, it is up to you to make further research on these to unveil something useful.

For example, we can have a company selling a huge number of products and they want to improve their system to target customers with useful advertisement campaigns. We can give to our algorithm the customers data and the algorithms can suggest some relations: age range, location, …

Reinforcement learning

In reinforcement learning algorithms, they do not receive immediately the reward for their actions, and they need to accumulate some consecutive decision to know if the actions/decisions are or not correct. In this scenario, there is no supervisor, the feedback about the decision is delayed and agent’s actions affect the subsequent data it receives.

One example of this, it can be the chess game, where the algorithm is going to be taking decisions but, till the end of the game, it is not going to be able to know if these decisions were correct or not and, obviously, previous decisions affect subsequent decisions.