Supervised learning: This is the most popular branch of machine learning, which is about learning from labeled data. If the data type of the label is categorical, it becomes a classification problem, and if numeric, it is known as a regression problem. For example, if the goal of using of the dataset is the detection of fraud, which has categorical values of either true or false, we are dealing with a classification problem. If, on the other hand, the target is to predict the best price to list the sale of a home, which is a numeric dollar value, the problem is one of regression. The following figure illustrates labeled data that warrants the use of classification techniques, such as logistic regression that is suitable for linearly separable data, that is, when there exists a line that can cleanly separate the two classes. For higher dimensional data that may be linearly separable, one speaks of a separating hyperplane:

Linearly separable data

An example of a dataset that is not linearly separable.

This type of problem calls for classification techniques, such as support vector machines.

Unsupervised learning: Understanding the data and exploring it for building machine learning models when the labels are not given is called unsupervised learning. Clustering, manifold learning, and outlier detection are techniques that are covered under this topic, which are dealt with in detail in Chapter 3, Unsupervised Machine Learning Techniques. Examples of problems that require unsupervised learning are many. Grouping customers according to their purchasing behavior is one example. In the case of biological data, tissues samples can be clustered based on similar gene expression values using unsupervised learning techniques.

The following figure represents data with inherent structure that can be revealed as distinct clusters using an unsupervised learning technique, such as k-means:

Clusters in data

Different techniques are used to detect global outliers—examples that are anomalous with respect to the entire dataset, and local outliers—examples that are misfits in their neighborhood. In the following figure, the notion of local and global outliers is illustrated for a two-feature dataset:

Local and global outliers

Semi-supervised learning: When the dataset has only some labeled data and a large amount of data that is not labeled, learning from such a dataset is called semi-supervised learning. When dealing with financial data with the goal of detecting fraud, for example, there may be a large amount of unlabeled data and only a small number of known fraud and non-fraud transactions. In such cases, semi-supervised learning may be applied.

Graph mining: Mining data represented as graph structures is known as graph mining. It is the basis of social network analysis and structure analysis in different bioinformatics, web mining, and community mining applications.

Probabilistic graph modeling and inferencing: Learning and exploiting conditional dependence structures present between features expressed as a graph-based model comes under the branch of probabilistic graph modeling. Bayesian networks and Markov random fields are two classes of such models.

Time-series forecasting: This refers to a form of learning where data has distinct temporal behavior and the relationship with time is modeled. A common example is in financial forecasting, where the performance of stocks in a certain sector may be the target of the predictive model.

Association analysis: This is a form of learning where data is in the form of an item set or market basket, and association rules are modeled to explore and predict the relationships between the items. A common example in association analysis is to learn the relationships between the most common items bought by customers when they visit the grocery store.

Reinforcement learning: This is a form of learning where machines learn to maximize performance based on feedback in the form of rewards or penalties received from the environment. A recent example that famously used reinforcement learning, among other techniques, was AlphaGo, the machine developed by Google that decisively beat the World Go Champion Lee Sedol in March 2016. Using a reward and penalty scheme, the model first trained on millions of board positions in the supervised learning stage, then played itself in the reinforcement learning stage to ultimately become good enough to triumph over the best human player.

Stream learning or incremental learning: Learning in a supervised, unsupervised, or semi-supervised manner from stream data in real time or pseudo-real time is called stream or incremental learning. Learning the behaviors of sensors from different types of industrial systems for categorizing into normal and abnormal is an application that needs real-time feeds and real-time detection.