Types of machine learning models: supervised vs unsupervised

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Supervised learning

Supervised learning is where we provide the model with the actual outputs from the data. This let’s it build a picture of the data and form links between the historic parameters (or features) that have influenced the output. To put a formula onto supervised learning, it would be as below, where, Y is the predicted output, produced by the model and X is the input data. So, by executing a function against X, we can predict Y.

Y = f(X)

The goal of supervised learning is to be able to model the influence of input parameters (X) so well, that we can accurately predict the output (Y).

Supervised learning is used for regression, classification, decision trees and neural networks, a few examples are included below:

  • Regression aims to predict the numeric value of something, given a set of input parameters. For example, the we could approximate the price of a car, given its mileage, age, brand, MOT status, etc..
  • Classification is about classifying things – for example, given the input parameters of an email (sender, subject line, email body), it should be classified as ‘spam’ or ‘not spam’.
  • Decision trees can be used for loan approval, based on income ranges, credit rating, criminal records and more

As supervised learning forms the bulk of machine learning projects in the real-world, I will be almost exclusively focussing on supervised learning models in the subsequent posts on Netshock. We will be looking at end-to-end projects, from definition, to feature extraction to modelling, to training to deployment.

Unsupervised learning

Unsupervised learning is where we do not provide the model with the actual outputs from the data. Unsupervised learning aims to model the underling structure or distribution in the data to learn more about the data. The most popular use-cases of unsupervised learning are association rules – which is where we uncover rules that describe a large chunk of our data. For example, Amazon uses such learning to state that people that bought this, also bought that.

Semi-supervised learning

Semi-supervised learning sits in the middle of supervised and unsupervised. It’s where only some of our input parameters have associated outputs. So, we don’t receive the actual result/output for given input parameters.

A good use-case for semi-supervised learning is web page classification. Let’s say we wish to classify web pages as ‘news’, ‘learning’, ‘entertainment’, etc… It’s very cheap and easy to crawl the web and extract a list of webpages, but it’s very expensive for humans to sit and classify them manually. So, we may choose to classify a sub-set of the data manually & use that to help train the model.

Summary

  • Supervised learning has the output for every input & we can use these to accurately train the model
  • Unsupervised learning has no outputs for any inputs & is left t0 find links / trends / patterns in the data
  • Semi-supervised has some outputs for some inputs and can have a mixture of supervised and unsupervised techniques applied to these problems.