Data science

Data science is the discipline of extracting actionable knowledge from data of various forms. The name data science emerged quite recently--it was invented by DJ Patil and Jeff Hammerbacher and popularized in the article Data Scientist: The Sexiest Job of the 21st Century in 2012. But the discipline itself had existed before for quite a while and previously was known by other names such as data mining or predictive analytics. Data science, like its predecessors, is built on statistics and machine learning algorithms for knowledge extraction and model building.

The science part of the term data science is no coincidence--if we look up science, its definition can be summarized to systematic organization of knowledge in terms testable explanations and predictions. This is exactly what data scientists do, by extracting patterns from available data, they can make predictions about future unseen data, and they make sure the predictions are validated beforehand. 

Nowadays, data science is used across many fields, including (but not limited to):

• Banking: Risk management (for example, credit scoring), fraud detection, trading
• Insurance: Claims management (for example, accelerating claim approval), risk and losses estimation, also fraud detection
• Health care: Predicting diseases (such as strokes, diabetes, cancer) and relapses
• Retail and e-commerce: Market basket analysis (identifying product that go well together), recommendation engines, product categorization, and personalized searches

This book covers the following practical use cases:

• Predicting whether an URL is likely to appear on the first page of a search engine
• Predicting how fast an operation will be completed given the hardware specifications
• Ranking text documents for a search engine
• Checking whether there is a cat or a dog on a picture
• Recommending friends in a social network
• Processing large-scale textual data on a cluster of computers

In all these cases, we will use data science to learn from data and use the learned knowledge to solve a particular business problem.

We will also use a running example throughout the book, building a search engine. We will use it to illustrate many data science concepts such as, supervised machine learning, dimensionality reduction, text mining, and learning to rank models. 

Machine learning

Machine learning is a part of computer science, and it is at the core of data science. The data itself, especially in big volumes, is hardly useful, but inside it hides highly valuable patterns. With the help of machine learning, we can recognize these hidden patterns, extract them, and then apply the learned information to the new unseen items. 

For example, given the image of an animal, a machine learning algorithm can say whether the picture is a dog or a cat; or, given the history of a bank client, it will say how likely the client is to default, that is, to fail to pay the debt.

Often, machine learning models are seen as black boxes that take in a data point and output a prediction for it. In this book, we will look at what is inside these black boxes and see how and when it is best to use them.

The typical problems that machine learning solves can be categorized in the following groups:

• Supervised learning: For each data point, we have a label--extra information that describes the outcome that we want to learn. In the cats versus dogs case, the data point is an image of the animal; the label describes whether it's a dog or a cat.
• Unsupervised learning: We only have raw data points and no label information is available. For example, we have a collection of e-mails and we would like to group them based on how similar they are. There is no explicit label associated with the e-mails, which makes this problem unsupervised.
• Semi-supervised learning: Labels are given only for a part of the data.
• Reinforcement learning: Instead of labels, we have a reward; something the model gets by interacting with the environment it runs in. Based on the reward, it can adapt and maximize it. For example, a model that learns how to play chess gets a positive reward each time it eats a figure of the opponent, and gets a negative reward each time it loses a figure; and the reward is proportional to the value of the figure. 

Supervised learning

As we discussed previously, for supervised learning we have some information attached to each data point, the label, and we can train a model to use it and to learn from it. For example, if we want to build a model that tells us whether there is a dog or a cat on a picture, then the picture is the data point and the information whether it is a dog or a cat is the label. Another example is predicting the price of a house--the description of a house is the data point, and the price is the label. 

We can group the algorithms of supervised learning into classification and regression algorithms based on the nature of this information.

In classification problems, the labels come from some fixed finite set of classes, such as {cat, dog}, {default, not default}, or {office, food, entertainment, home}. Depending on the number of classes, the classification problem can be binary (only two possible classes) or multi-class (several classes).

Examples of classification algorithms are Naive Bayes, logistic regression, perceptron, Support Vector Machine (SVM), and many others. We will discuss classification algorithms in more detail in the first part of Chapter 4, Supervised Learning - Classification and Regression.

In regression problems, the labels are real numbers. For example, a person can have a salary in the range from $0 per year to several billions per year. Hence, predicting the salary is a regression problem.

Examples of regression algorithms are linear regression, LASSO, Support Vector Regression (SVR), and others. These algorithms will be described in more detail in the second part of Chapter 4, Supervised Learning - Classification and Regression.

Some of the supervised learning methods are universal and can be applied to both classification and regression problems. For example, decision trees, random forest, and other tree-based methods can tackle both types. We will discuss one such algorithm, gradient boosting machines in Chapter 7, Extreme Gradient Boosting.

Neural networks can also deal with both classification and regression problems, and we will talk about them in Chapter 8, Deep Learning with DeepLearning4J.

Unsupervised learning

Unsupervised learning covers the cases where we have no labels available, but still want to find some patterns hidden in the data. There are several types of unsupervised learning, and we will look into cluster analysis, or clustering and unsupervised dimensionality reduction. 

Clustering

Typically, when people talk about unsupervised learning, they talk about cluster analysis or clustering. A cluster analysis algorithm takes a set of data points and tries to categorize them into groups such that similar items belong to the same group, and different items do not. There are many ways where it can be used, for example, in customer segmentation or text categorization.

Customer segmentation is an example of clustering. Given some description of customers, we try to put them into groups such that the customers in one group have similar profiles and behave in a similar way. This information can be used to understand what do the people in these groups want, and this can be used to target them with better advertisements and other promotional messages.

Another example is text categorization. Given a collection of texts, we would like to find common topics among these texts and arrange the texts according to these topics. For example, given a set of complaints in an e-commerce store, we may want to put ones that talk about similar things together, and this should help the users of the system navigate through the complaints easier.

Examples of cluster analysis algorithms are hierarchical clustering, k-means, density-based spatial clustering of applications with noise (DBSCAN), and many others. We will talk about clustering in detail in the first part of Chapter 5, Unsupervised Learning - Clustering and Dimensionality Reduction.

Dimensionality reduction

Another group of unsupervised learning algorithms is dimensionality reduction algorithms. This group of algorithms compresses the dataset, keeping only the most useful information. If our dataset has too much information, it can be hard for a machine learning algorithm to use all of it at the same time. It may just take too long for the algorithm to process all the data and we would like to compress the data, so processing it takes less time. 

There are multiple algorithms that can reduce the dimensionality of the data, including Principal Component Analysis (PCA), Locally linear embedding, and t-SNE. All these algorithms are examples of unsupervised dimensionality reduction techniques.

Not all dimensionality reduction algorithms are unsupervised; some of them can use labels to reduce the dimensionality better. For example, many feature selection algorithms rely on labels to see what features are useful and what are not. 

We will talk more about this in Chapter 5, Unsupervised Learning - Clustering and Dimensionality Reduction.

Natural Language Processing

Processing natural language texts is very complex, they are not very well structured and require a lot of cleaning and normalizing. Yet the amount of textual information around us is tremendous: a lot of text data is generated every minute, and it is very hard to retrieve useful information from them. Using data science and machine learning is very helpful for text problems as well; they allow us to find the right text, process it, and extract the valuable bits of information.

There are multiple ways we can use the text information. One example is information retrieval, or, simply, text search--given a user query and a collection of documents, we want to find what are the most relevant documents in the corpus with respect to the query, and present them to the user. Other applications include sentiment analysis--predicting whether a product review is positive, neutral or negative, or grouping the reviews according to how they talk about the products. 

We will talk more about information retrieval, Natural Language Processing (NLP) and working with texts in Chapter 6, Working with Text - Natural Language Processing and Information Retrieval. Additionally, we will see how to process large amounts of text data in Chapter 9, Scaling Data Science.  

The methods we can use for machine learning and data science are very important. What is equally important is the the way we create them and then put them to use in production systems. Data science process models help us make it more organized and systematic, which is why we will talk about them next.