Most of us are unaware that we already interact with Machine Learning every single day. Every time we Google something, listen to a song or even take a photo, Machine Learning is becoming part of the engine behind it, constantly learning and improving from every interaction. It’s also behind world-changing advances like detecting cancer, creating new drugs and self-driving cars.
Traditionally, software engineering combined human created rules with data to create answers to a problem. Instead, machine learning uses data and answers to discover the rules behind a problem.
To learn the rules governing a phenomenon, machines have to go through a learning process, trying different rules and learning from how well they perform. Hence, why it’s known as Machine Learning.
There are multiple forms of Machine Learning; supervised, unsupervised, semi-supervised and reinforcement learning. Each form of Machine Learning has differing approaches, but they all follow the same underlying process and theory. This explanation covers the general Machine Learning concept and then focuses in on each approach.
In supervised learning, the goal is to learn the mapping (the rules) between a set of inputs and outputs.
For example, the inputs could be the weather forecast, and the outputs would be the visitors to the beach. The goal in supervised learning would be to learn the mapping that describes the relationship between temperature and number of beach visitors.
Example labelled data is provided of past input and output pairs during the learning process to teach the model how it should behave, hence, ‘supervised’ learning. For the beach example, new inputs can then be fed in of forecast temperature and the Machine learning algorithm will then output a future prediction for the number of visitors.
Being able to adapt to new inputs and make predictions is the crucial generalisation part of machine learning. In training, we want to maximise generalisation, so the supervised model defines the real ‘general’ underlying relationship. If the model is over-trained, we cause over-fitting to the examples used and the model would be unable to adapt to new, previously unseen inputs.
A side effect to be aware of in supervised learning that the supervision we provide introduces bias to the learning. The model can only be imitating exactly what it was shown, so it is very important to show it reliable, unbiased examples. Also, supervised learning usually requires a lot of data before it learns. Obtaining enough reliably labelled data is often the hardest and most expensive part of using supervised learning. (Hence why data has been called the new oil!)
The output from a supervised Machine Learning model could be a category from a finite set e.g [low, medium, high] for the number of visitors to the beach:
Input [temperature=20] -> Model -> Output = [visitors=high]
When this is the case, it’s is deciding how to classify the input, and so is known as classification.
Alternatively, the output could be a real-world scalar (output a number):
Input [temperature=20] -> Model -> Output = [visitors=300]
When this is the case, it is known as regression.
Classification is used to group the similar data points into different sections in order to classify them. Machine Learning is used to find the rules that explain how to separate the different data points.
But how are the magical rules created? Well, there are multiple ways to discover the rules. They all focus on using data and answers to discover rules that linearly separate data points.
Linear separability is a key concept in machine learning. All that linear separability means is ‘can the different data points be separated by a line?’. So put simply, classification approaches try to find the best way to separate data points with a line.
The lines drawn between classes are known as the decision boundaries. The entire area that is chosen to define a class is known as the decision surface. The decision surface defines that if a data point falls within its boundaries, it will be assigned a certain class.
Regression is another form of supervised learning. The difference between classification and regression is that regression outputs a number rather than a class. Therefore, regression is useful when predicting number based problems like stock market prices, the temperature for a given day, or the probability of an event.
In unsupervised learning, only input data is provided in the examples. There are no labelled example outputs to aim for. But it may be surprising to know that it is still possible to find many interesting and complex patterns hidden within data without any labels.
An example of unsupervised learning in real life would be sorting different colour coins into separate piles. Nobody taught you how to separate them, but by just looking at their features such as colour, you can see which colour coins are associated and cluster them into their correct groups.
Unsupervised learning can be harder than supervised learning, as the removal of supervision means the problem has become less defined. The algorithm has a less focused idea of what patterns to look for.
Think of it in your own learning. If you learnt to play the guitar by being supervised by a teacher, you would learn quickly by re-using the supervised knowledge of notes, chords and rhythms. But if you only taught yourself, you’d find it so much harder knowing where to start.
By being unsupervised in a laissez-faire teaching style, you start from a clean slate with less bias and may even find a new, better way solve a problem. Therefore, this is why unsupervised learning is also known as knowledge discovery. Unsupervised learning is very useful when conducting exploratory data analysis.
To find the interesting structures in unlabeled data, we use density estimation. The most common form of which is clustering. Among others, there is also dimensionality reduction, latent variable models and anomaly detection. More complex unsupervised techniques involve neural networks like Auto-encoders and Deep Belief Networks, but we won’t go into them in this introduction blog.
Unsupervised learning is mostly used for clustering. Clustering is the act of creating groups with differing characteristics. Clustering attempts to find various subgroups within a dataset. As this is unsupervised learning, we are not restricted to any set of labels and are free to choose how many clusters to create. This is both a blessing and a curse. Picking a model that has the correct number of clusters (complexity) has to be conducted via an empirical model selection process.
Dimensionality reduction aims to find the most important features to reduce the original feature set down into a smaller more efficient set that still encodes the important data.
For example, in predicting the number of visitors to the beach we might use the temperature, day of the week, month and number of events scheduled for that day as inputs. But the month might actually be not important for predicting the number of visitors.
Irrelevant features such as this can confuse a Machine Leaning algorithms and make them less efficient and accurate. By using dimensionality reduction, only the most important features are identified and used. Principal Component Analysis (PCA) is a commonly used technique.