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Machine Learning Key Terms, Explained

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An overview of 12 important machine learning concepts, presented in a no frills, straightforward definition style.

6. Decision Trees
Decision trees are top-down, recursive, divide-and-conquer classifiers. Decision trees are generally composed of 2 main tasks: tree induction and tree pruning. Tree induction is the task of taking a set of pre-classified instances as input, deciding which attributes are best to split on, splitting the dataset, and recursing on the resulting split datasets until all training instances are categorized. While building our tree, the goal is to split on the attributes which create the purest child nodes possible, which would keep to a minimum the number of splits that would need to be made in order to classify all instances in our dataset. This purity is measured by the concept of information, which relates to how much would need to be known about a previously-unseen instance in order for it to be properly classified.

A completed decision tree model can be overly-complex, contain unnecessary structure, and be difficult to interpret. Tree pruning is the process of removing the unnecessary structure from a decision tree in order to make it more efficient, more easily-readable for humans, and more accurate as well. This increased accuracy is due to pruning’s ability to reduce overfitting.

7. Support Vector Machines
SVMs are able to classify both linear and nonlinear data. SMVs work by transforming the training dataset into a higher dimension, a higher dimension which is then inspected for the optimal separation boundary, or boundaries, between classes. In SVMs, these boundaries are referred to as hyperplanes, which are identified by locating support vectors, or the instances that most essentially define classes, and their margins, which are the lines parallel to the hyperplane defined by the shortest distance between a hyperplane and its support vectors.

The grand idea with SVMs is that, with a high enough number of dimensions, a hyperplane separating 2 classes can always be found, thereby delineating dataset member classes. When repeated a sufficient number of times, enough hyperplanes can be generated to separate all classes in n-dimension space.


8. Neural Networks
Neural networks are algorithms inspired by the biological brain, although the extent to which they capture actual brain functionality is highly controversial, and claims that they model the biological brain are patently false. Neural networks are made up of numerous interconnected conceptualized artificial neurons, which pass data between themselves, and which have associated weights which are tuned based upon the newtork's "experience." Neurons have activation thresholds which, if met by a combination of their associated weights and data passed to them, are fired; combinations of fired neurons result in "learning."

9. Deep Learning
Deep learning is a relatively new term, although it has existed prior to the dramatic uptick in online searches of late. Enjoying a surge in research and industry, due mainly to its incredible successes in a number of different areas, deep learning is the process of applying deep neural network technologies - that is, neural network architectures with multiple hidden layers of neurons - to solve problems. Deep learning is a process, like data mining, which employs deep neural network architectures, which are particular types of machine learning algorithms.

Deep learning

10. Reinforcement Learning
Bishop best describes reinforcement learning in a single concise sentence: "Reinforcement learning is concerned with the problem of finding suitable actions to take in a given situation in order to maximize a reward." Reinforcement algorithms are not given explicit goals; instead, they are forced to learn these optimal goals by trial and error. Think of the classic Mario Bros. video game; reinforcement learning algorithms would, by trial and error, determine that certain movements and button pushes would advance the player's standing in the game, and trial and error would aim to result in an optimal state of game play.

11. (k-fold) Cross-validation
Cross-validation is a deterministic method for model building, achieved by leaving out one of k segments, or folds, of a dataset, training on all k-1 segments, and using the remaining kth segment for testing; this process is then repeated k times, with the individual prediction error results being combined and averaged in a single, integrated model. This provides variability, with the goal of producing the most accurate predictive models possible.

12. Bayesian
When referring to probability, there are 2 major schools of thought: classical, or frequentist, probability interpretation views probabilities in terms of the frequencies of random events. In somewhat of a contrast, the Bayesian view of probability aims to quantify uncertainty, and updates a given probability as additional evidence is available. If these probabilities are extended to truth values, and are assigned to hypotheses, we then have "learning" to varying degrees of certainty.


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