Machine learning (ML) is the most common technique for predicting the future or for classifying information, to help people make the required decisions. ML techniques are based on algorithms – sets of mathematical procedures that explain the relationship between variables. ML algorithms are trained in situations where they learn from past data and can even evaluate historical data. After extensive training, the algorithm can recognize patterns sufficiently to make predictions. ML provides methods, techniques, and tools that can help in solving diagnostic and prognostic problems in a variety of medical domains. It is being used for the analysis of the importance of clinical parameters and their combinations for prognosis, e.g., for prediction of disease progression, for the extraction of medical information to predict research outcomes, for therapy planning and support, and for overall patient management. ML is also being used for data analysis, such as detection of patterns in the data by dealing appropriately with imperfect data, interpretation of continuous data generated in the Intensive Care Unit, and for intelligent alarms, resulting in effective and efficient monitoring. Successful implementation of ML methods can help the integration of computer-based systems into the healthcare environment, providing opportunities to facilitate and enhance the work of medical experts, and ultimately to improve the efficiency and quality of medical care. Most ML methods can be categorized into one of two types of learning techniques, namely supervised or unsupervised algorithms. A supervised learning algorithm takes a known set of input data (the training set) and known responses to the data (the output) and trains a model to generate reasonable predictions for the response to new input data. In medicine, supervised learning involves training a model to relate a person’s characteristics (e.g., height, weight, smoking status) to a certain outcome (onset of diabetes within five years, for example). Once the algorithm is successfully trained, it will be capable of making outcome predictions when supplied with new data. Predictions which are made by models trained using supervised learning can be either discrete (e.g., positive or negative, benign or malignant) or continuous (e.g., a score from 0 to 100). A supervised model which produces discrete categories (sometimes referred to as classes) is referred to as a classification algorithm. Examples of classification algorithms include those which predict whether a tumor is benign or malignant, or to establish whether comments written by a patient convey a positive or negative sentiment. In practice, classification algorithms return the probability of a class (between 0 for impossible and 1 for certain). Common classification algorithms are Logistic Regression, K-Nearest Neighbor, Support Vector Machine, Neural Network, and Naïve Bayes. A supervised model which returns a prediction of a continuous value is known as a regression algorithm, which might be used by ML to predict an individual’s life expectancy or the tolerable dose of chemotherapy. Supervised ML algorithms are typically developed using a dataset, which contains several variables and a relevant outcome. In contrast with supervised learning, unsupervised learning does not involve a predefined outcome. In unsupervised learning, patterns are sought by algorithms without any input from the user. Unsupervised techniques are thus exploratory and used to find undefined patterns or clusters which occur within datasets. In this chapter, the use of different types of supervised and unsupervised ML algorithms for various applications in health care are discussed.

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