How machines learn

A formal definition of machine learning attributed to computer scientist Tom M. Mitchell states that a machine learns whenever it is able to utilize its experience such that its performance improves on similar experiences in the future. Although this definition is intuitive, it completely ignores the process of exactly how experience can be translated into future action—and, of course, learning is always easier said than done!

Where human brains are naturally capable of learning from birth, the conditions necessary for computers to learn must be made explicit. For this reason, although it is not strictly necessary to understand the theoretical basis of learning, this foundation helps us to understand, distinguish, and implement machine learning algorithms.

Tip

As you compare machine learning to human learning, you may find yourself examining your own mind in a different light.

Regardless of whether the learner is a human or a machine, the basic learning process is similar. It can be divided into four interrelated components:

• Data storage utilizes observation, memory, and recall to provide a factual basis for further reasoning.
• Abstraction involves the translation of stored data into broader representations and concepts.
• Generalization uses abstracted data to create knowledge and inferences that drive action in new contexts.
• Evaluation provides a feedback mechanism to measure the utility of learned knowledge and inform potential improvements.

Figure 1.4: The learning process

Although the learning process has been conceptualized here as four distinct components, they are merely organized this way for illustrative purposes. In reality, the entire learning process is inextricably linked. In human beings, the process occurs subconsciously. We recollect, deduce, induct, and intuit within the confines of our mind's eye, and because this process is hidden, any differences from person to person are attributed to a vague notion of subjectivity. In contrast, computers make these processes explicit, and because the entire process is transparent, the learned knowledge can be examined, transferred, utilized for future action, and treated as a data "science."

The data science buzzword suggests a relationship among the data, the machine, and the people who guide the learning process. The term's growing use in job descriptions and academic degree programs reflects its operationalization as a field of study concerned with both statistical and computational theory, as well as the technological infrastructure enabling machine learning and its applications. The field often asks its practitioners to be compelling storytellers, balancing an audacity in the use of data with the limitations of what one may infer and forecast from the data. To be a strong data scientist, therefore, requires a strong understanding of how the learning algorithms work.

Data storage

All learning begins with data. Humans and computers alike utilize data storage as a foundation for more advanced reasoning. In a human being, this consists of a brain that uses electrochemical signals in a network of biological cells to store and process observations for short- and long-term future recall. Computers have similar capabilities of short- and long-term recall using hard disk drives, flash memory, and random-access memory (RAM) in combination with a central processing unit (CPU).

It may seem obvious, but the ability to store and retrieve data alone is insufficient for learning. Stored data is merely ones and zeros on a disk. It is a collection of memories, meaningless without a broader context. Without a higher level of understanding, knowledge is purely recall, limited to what has been seen before and nothing else.

To better understand the nuances of this idea, it may help to think about the last time you studied for a difficult test, perhaps for a university final exam or a career certification. Did you wish for an eidetic (photographic) memory? If so, you may be disappointed to know that perfect recall would unlikely be of much assistance. Even if you could memorize material perfectly, this rote learning would provide no benefit without knowing the exact questions and answers that would appear on the exam. Otherwise, you would need to memorize answers to every question that could conceivably be asked, on a subject in which there is likely to be an infinite number of questions. Obviously, this is an unsustainable strategy.

Instead, a better approach is to spend time selectively, and memorize a relatively small set of representative ideas, while developing an understanding of how the ideas relate and apply to unforeseen circumstances. In this way, important broader patterns are identified, rather than memorizing each and every detail, nuance, and potential application.

Abstraction

This work of assigning a broader meaning to stored data occurs during the abstraction process, in which raw data comes to represent a wider, more abstract concept or idea. This type of connection, say between an object and its representation, is exemplified by the famous René Magritte painting The Treachery of Images:

Figure 1.5: "This is not a pipe." Source: http://collections.lacma.org/node/239578

The painting depicts a tobacco pipe with the caption Ceci n'est pas une pipe ("This is not a pipe"). The point Magritte was illustrating is that a representation of a pipe is not truly a pipe. Yet, in spite of the fact that the pipe is not real, anybody viewing the painting easily recognizes it as a pipe. This suggests that observers are able to connect the picture of a pipe to the idea of a pipe, to a memory of a physical pipe that can be held in the hand. Abstracted connections like this are the basis of knowledge representation, the formation of logical structures that assist with turning raw sensory information into meaningful insight.

During a machine's process of knowledge representation, the computer summarizes stored raw data using a model, an explicit description of the patterns within the data. Just like Magritte's pipe, the model representation takes on a life beyond the raw data. It represents an idea greater than the sum of its parts.

There are many different types of models. You may already be familiar with some. Examples include:

• Mathematical equations
• Relational diagrams, such as trees and graphs
• Logical if/else rules
• Groupings of data known as clusters

The choice of model is typically not left up to the machine. Instead, the learning task and the type of data on hand inform model selection. Later in this chapter, we will discuss in more detail the methods for choosing the appropriate model type.

The process of fitting a model to a dataset is known as training. When the model has been trained, the data has been transformed into an abstract form that summarizes the original information.

Tip

You might wonder why this step is called "training" rather than "learning." First, note that the process of learning does not end with data abstraction—the learner must still generalize and evaluate its training. Second, the word "training" better connotes the fact that the human teacher trains the machine student to understand the data in a specific way.

It is important to note that a learned model does not itself provide new data, yet it does result in new knowledge. How can this be? The answer is that imposing an assumed structure on the underlying data gives insight into the unseen. It supposes a new concept that describes a manner in which data elements may be related.

Take, for instance, the discovery of gravity. By fitting equations to observational data, Sir Isaac Newton inferred the concept of gravity, but the force we now know as gravity was always present. It simply wasn't recognized until Newton expressed it as an abstract concept that relates some data to other data—specifically, by becoming the g term in a model that explains observations of falling objects.

Figure 1.6: Models are abstractions that explain observed data

Most models will not result in the development of theories that shake up scientific thought for centuries. Still, your abstraction might result in the discovery of important, but previously unseen, patterns and relationships among data. A model trained on genomic data might find several genes that when combined are responsible for the onset of diabetes, banks might discover a seemingly innocuous type of transaction that systematically appears prior to fraudulent activity, or psychologists might identify a combination of personality characteristics indicating a new disorder. These underlying patterns were always present, but by presenting information in a different format, a new idea is conceptualized.

Generalization

The next step in the learning process is to use the abstracted knowledge for future action. However, among the countless underlying patterns that may be identified during the abstraction process and the myriad ways to model those patterns, some patterns will be more useful than others. Unless the production of abstractions is limited to the useful set, the learner will be stuck where it started, with a large pool of information but no actionable insight.

Formally, the term generalization is defined as the process of turning abstracted knowledge into a form that can be utilized for future action, on tasks that are similar, but not identical, to those the learner has seen before. It acts as a search through the entire set of models (that is, theories or inferences) that could be established from the data during training.

If you can imagine a hypothetical set containing every possible way the data might be abstracted, generalization involves the reduction of this set into a smaller and more manageable set of important findings.

In generalization, the learner is tasked with limiting the patterns it discovers to only those that will be most relevant to its future tasks. Normally, it is not feasible to reduce the number of patterns by examining them one-by-one and ranking them by future utility. Instead, machine learning algorithms generally employ shortcuts that reduce the search space more quickly. To this end, the algorithm will employ heuristics, which are educated guesses about where to find the most useful inferences.

Tip

Heuristics utilize approximations and other rules of thumb, which means they are not guaranteed to find the best model of the data. However, without taking these shortcuts, finding useful information in a large dataset would be infeasible.

Heuristics are routinely used by human beings to quickly generalize experience to new scenarios. If you have ever utilized your gut instinct to make a snap decision prior to fully evaluating your circumstances, you were intuitively using mental heuristics.

The incredible human ability to make quick decisions often relies not on computer-like logic, but rather on emotion-guided heuristics. Sometimes, this can result in illogical conclusions. For example, more people express fear of airline travel than automobile travel, despite automobiles being statistically more dangerous. This can be explained by the availability heuristic, which is the tendency for people to estimate the likelihood of an event by how easily examples can be recalled. Accidents involving air travel are highly publicized. Being traumatic events, they are likely to be recalled very easily, whereas car accidents barely warrant a mention in the newspaper.

The folly of misapplied heuristics is not limited to human beings. The heuristics employed by machine learning algorithms also sometimes result in erroneous conclusions. The algorithm is said to have a bias if the conclusions are systematically erroneous, which implies that they are wrong in a consistent or predictable manner.

For example, suppose that a machine learning algorithm learned to identify faces by finding two dark circles representing eyes, positioned above a straight line indicating a mouth. The algorithm might then have trouble with, or be biased against, faces that do not conform to its model. Faces with glasses, turned at an angle, looking sideways, or with certain skin tones might not be detected by the algorithm. Similarly, it could be biased toward faces with other skin tones, face shapes, or characteristics that conform to its understanding of the world.

Figure 1.7: The process of generalizing a learner's experience results in a bias

In modern usage, the word "bias" has come to carry quite negative connotations. Various forms of media frequently claim to be free from bias, and claim to report the facts objectively, untainted by emotion. Still, consider for a moment the possibility that a little bias might be useful. Without a bit of arbitrariness, might it be a little difficult to decide among several competing choices, each with distinct strengths and weaknesses? Indeed, studies in the field of psychology have suggested that individuals born with damage to the portions of the brain responsible for emotion may be ineffectual at decision-making and might spend hours debating simple decisions, such as what color shirt to wear or where to eat lunch. Paradoxically, bias is what blinds us from some information, while also allowing us to utilize other information for action. It is how machine learning algorithms choose among the countless ways to understand a set of data.

Evaluation

Bias is a necessary evil associated with the abstraction and generalization processes inherent in any learning task. In order to drive action in the face of limitless possibility, all learning must have a bias. Consequently, each learning strategy has weaknesses; there is no single learning algorithm to rule them all. Therefore, the final step in the learning process is to evaluate its success, and to measure the learner's performance in spite of its biases. The information gained in the evaluation phase can then be used to inform additional training if needed.

Tip

Once you've had success with one machine learning technique, you might be tempted to apply it to every task. It is important to resist this temptation because no machine learning approach is best for every circumstance. This fact is described by the No Free Lunch theorem, introduced by David Wolpert in 1996. For more information, visit: http://www.no-free-lunch.org.

Generally, evaluation occurs after a model has been trained on an initial training dataset. Then, the model is evaluated on a separate test dataset in order to judge how well its characterization of the training data generalizes to new, unseen cases. It's worth noting that it is exceedingly rare for a model to perfectly generalize to every unforeseen case—mistakes are almost always inevitable.

In part, models fail to generalize perfectly due to the problem of noise, a term that describes unexplained or unexplainable variations in data. Noisy data is caused by seemingly random events, such as:

• Measurement error due to imprecise sensors that sometimes add or subtract a small amount from the readings
• Issues with human subjects, such as survey respondents reporting random answers to questions in order to finish more quickly
• Data quality problems, including missing, null, truncated, incorrectly coded, or corrupted values
• Phenomena that are so complex or so little understood that they impact the data in ways that appear to be random

Trying to model noise is the basis of a problem called overfitting; because most noisy data is unexplainable by definition, attempting to explain the noise will result in models that do not generalize well to new cases. Efforts to explain the noise also typically result in more complex models that miss the true pattern the learner is trying to identify.

Figure 1.8: Modeling noise generally results in more complex models and misses underlying patterns

A model that performs relatively well during training but relatively poorly during evaluation is said to be overfitted to the training dataset because it does not generalize well to the test dataset. In practical terms, this means that it has identified a pattern in the data that is not useful for future action; the generalization process has failed. Solutions to the problem of overfitting are specific to particular machine learning approaches. For now, the important point is to be aware of the issue. How well the methods are able to handle noisy data and avoid overfitting is an important point of distinction among them.