# Machine Learning¶

Learning Objectives

• Learn what machine learning is

• Understand the strength and limitations

• Distinguish unsupervised learning, supervised learning, reinforcement learning, and transfer learning

## Decision Problems¶

Machine learning means using computer programs to discover patterns (i.e., learn) from data. How is this different from typical computer programs?

### Bring an Umbrella or Not¶

Let us consider a “decision problem”: Should you bring an umbrella or not? This should be easy, right? You can decide based on “whether it is raining or not”. This is simple enough. If you always follow this rule, you will soon encounter situations when this rule is not good. Maybe it is not raining right now but it is expected to rain soon. Thus, you add one more rule: “Bring an umbrella if it is raining or it is expected to rain.”. If you follow these rules, you will soon discover that they are still insufficient. If you are going to walk in covered areas (such as subway stations or shopping malls), maybe you do not need an umbrella. Thus, you add another rule: “Bring an umbrella if it is raining or it is expected to rain and you will walk in an uncovered area.” Things can become really complicated. If you are carrying a box and it does not rain heavily, maybe you do not want to take an umbrella because it is inconvenient to carry the box and to hold an umbrella at the same time. As a result, the rules become “Bring an umbrella if it is raining or it is expected to rain and you will walk in an uncovered area. Do not bring an umbrealla if it does not rain heavily and you are carrying a box.” The rules become more and more complex. You may also want to consider whether it is windy. If it is, you may want to wear a raincoat instead of bringing an umbrella. If you want to ride a bike, you may want to choose a rain coat instead of an umbrella. Maybe it is very hot and you want to walk in rain to cool. As you can see, this decision problem has so many different scenarios and writing all these as if-else conditions becomes really complex.

### Cross a Stree Intersection¶

If you drive a car, should you cross a street intersection or not? You may think this is really easy: “If the traffic light is green, cross the intersection.” Sometimes, you cannot enter the inserction because it is blocked by vehicles already. If you enter the intersection, you will park at the intersection and worsen traffic jam. Thus, you add another rule: “If the traffic light is green and the intersection is not blocked, cross the intersection.” If you hear the siren of an ambulance, you should allow it to pass first. You add another condition: “If the traffic light is green and the intersection is not blocked and there is no siren, cross the intersection.” If there is a jaywalker, you don’t want to hit the person. You add another rule: “If the traffic light is green and the intersection is not blocked and there is no siren and there is no jaywalker, cross the intersection.” If there is a construction and a flagman, you should follow the flagman’s instruction, not the traffic light. You can keep adding more and more rules to cover many different scenarios. This decision problem may consider many factors and writing down these rules become increasingly complex and difficult.

### Approve a Mortgate Application¶

If you are a bank manager and evaluate mortgage applications, how do you decide whether to approve or not? If you approve the application and the person pays regularly, your bank makes money from fees and interests. If the person fails to pay (called “defaults”), the bank may lose money. It is possible that the bank does not lose money if the house’s value is sufficient to cover the mortgage (through foreclosure). Foreclosure can be a lengthy process and most banks want to avoid it. How do you decide? Maybe you decide based on the person’s regular income: “* If an applicant’s monthly income is more than twice the mortgage payment, approve the application.*” If you do this, your bank will likely lose a lot of money because you need to consider other factors, for example, whether the person already has a lot of debt. You probably also want to consider whether this person has a record of failing to pay bills. If the house is not in a popular location, you definitely want to avoid the possibility of foreclosure. If the person has been doing business with the bank for several years, you might trust this person more than a new customer. Do you want to consider whether this person already owns one or more houses? Should you consider this person’s age? Would the marital status affect your decision? You may want to consider the economy as well. If economy is strong, this applicant is likely to keep the current job or even get a raise; thus, this applicant is likely to pay the mortgage. If economy is weak, this applicant may lose the current job and fails to pay. If you want to consider many factors, are some factors are more important than the others? How do you choose the important factors? This problem, again, shows that considering many factors and writing down the rules become increasingly complex and difficult.

## Decisions and Feedback¶

In our everyday life, we make hundreds of decisions: what clothes to wear, where to go for lunch, what to buy in a store, what birthday gift to send to a friend, etc. The previous examples show that decision problems often need to consider many factors. The reason we need to consider many factors is to prevent making wrong decisions. What are “wrong” decisions? If you bring an umbrella and it does not rain, is it a wrong decision? If you do not bring an umbrella and it rain heavily, it is a wrong decision. Some wrong decisions have negligible consequences: bringing an umbrella (if it is small and light) without using it may not be a big problem. Some wrong decisions can have dire consequences: approving a mortgage and it defaults, the bank loses a lot of money. The definition of “wrong” decisions may not be so obvious. A bank may deny all mortgage applications that have slight chances of defaults. This may completely avoid defaults but the bank also loses opportunities making money from the applications that may, but do not, default. In this case, “preventing defaults” and “making money” are two related but different goals. After knowing the result of a decision, we may conclude that it is a right or a wrong decision. This is the “feedback” of the decision. With the feedback, we hope to make better decisions in the future. For example, if the bank approves a mortgage and it defaults, the bank would probably deny the next application that is “similar” to the defaulted one. The problem is how to determine two mortgage applications are “similar”. For some problems, it is impossible (or almost impossible) to know whether a wrong decision has been made. Suppose you are the bank manager and you deny a mortgage application. You will not know whether this person would be able to pay mortgage regularly because you have denied the application.

## Knowledge in Data¶

The examples described earlier are decision problems: whether to bring an umbrella, whether to enter a traffic intersection, whether to approve a mortgage application, etc. Each problem needs to consider many factors and it is not always clear which factor is more important than the other. One way to solve these problems is to examine similar scenarios in the past and their results: if the current mortgage application is “similar” to one that was approved in the past, was that approval a right decisoin (i.e., did not default)? Instead of writing rules, this new approach uses past data to guide future decisions. This is what machine learning can be helpful: computer programs discover (i.e., “learn”) patterns from data. Past data may help decide which factors are important for making decisions if the past data and the new decision problem have similar patterns. Having similar patterns is an essential assumption in machine learning. Think about how humans learn: a person observes something and then when the person sees similar problems, the person uses past knowledge and experience to infer or derive the solutions for the new problems. If a person has never seen anything similar, the person would not able to draw from past knowledge or experience.

Machine learning relies on the assumption that past observations and new, unseen, situations have similar patterns. This assumption is essential to the success of machine learning. Imagine that you are the bank manager and have discovered a good way to determine whether to approve or deny mortgage applications. If you move to another city or another country, your method may be wrong more often than you expect. Maybe the demographics are different. Maybe the cultural norms are different. Maybe the real estate markets are different. This indicates that your machine learning method has its limitations. You may need to add some more data into building your knowledge about the new problem.

There are different types of learning: Supervised learning means that there is a “teacher” telling a student what is right or wrong. Imagine that a teacher shows images of flowers and tell students that these are flowers. The teacher shows another image of an elephant and says that it is an elephant. Unsupervised learning has no teacher. Imagine that you want to stock your store on a Friday evening for sales on Saturday. There is no correct answer what products you should put on shelves. You can analyze the past sales records, together with factors such as weather and season. You may also want to consider whether there is a major sport event on that Saturday. This is different from supervised learning because there is no teacher telling you “Yes, you should stock this item on shelves.” or “No, do not stock that item because few people will buy it this coming Saturday.” Unsupervised learning is often used to discover (unknown) properties in data, for example, what people buy on a Saturday. The third type of learning is called reinforcement learning. It considers sequences of actions (such as moves in chess) and the rewards (such as winning a chess game) of these actions. Reinforcement learning is different from supervised learning because most decisions cannot be consider right or wrong. Some decisions such as checkmate are obviously right decisions but the effects of most decisions are unkonwn until much later. Instead, the sequence of decisions leads to a result, either winning or losing. Reinforcement learning is usually used for developing strategies solving problems through sequences of actions. The fourth type of learning is called transfer learning. The knowledge learned from the sample data is “transferred” to a new set of test data. An analogy is that a person learns English and then uses the knowelege about sentence structures and tenses to learn French.

Supervised learning may be the most familar form of learning: babies learn parents’ faces when the parents say “Daddy” and “Mommy”. Students learn from teachers in classrooms. Supervised learning, however, can be expensive because teachers are needed. As computer technologies improve, acquiring data becomes very easy and inexpensive. Spending \$100, you can buy a video camera and the it can easily generate thousands of images (more precisely, video frames) per day. Teachig computer the information in the images requires humans as teachers because computer programs cannot perfectly analyze images yet. Teaching computers by marking what is in the images is called labeling or annnotating. In some cases, labeling can be crowdsourced. Labeling one million images by humans would not be easy. In some other cases, the “teachers” of computers must have special qualifications; for example, medical images are evaluated by trained medical doctors.

Unsupervised learning can be applied when a person mimicks the behaviors of another person. Imagine that you have a vacation in USA and hear people saying “hello” when they meet. Even though nobody (i.e., there is no teacher) tells you what it means, you start saying “hello” when you meet people. This is an example of unsupervised learning. Unsupervised learning can also be used to discover patterns in data, for example, people that buy apples are liekly to buy organes also. Web search engines are examples of unsupervised learning. These engines analyze many websites and rank websites for different search keywords. There is no “teacher” specifying the correct orders.

## Define Learning¶

We have talked about “learning” without actually defining it. What is learning? In Machine Learning by Tom M. Mitchell, machine learning is defined as

A computer program is said to learn from experience E with respect to some class of tasks T and performance measure P, if its performance at tasks in T, as measured by P, improves with experience E.

To explain this in a more intuitive way, a computer program can learn if it can get better by doing something more. One way to understand learning is by comparing it with something that cannot learn. Consider the calculator program on your mobile phone. It does not get better after you use it. In contrast, a program that determines whether an email is spam may get better after you mark some emails as spam. By marking spam emails, you play the role of a teacher and this is an example of supervised learning.

This definition does not speficy what is “experience”. From computers’ viewpoint, the experience often refers to “data”. If more data is used (assuming the data follows specific patterns), then the computer program can perform better (such as making more correct decisions in mortgage applications). What is machine learning really? Machine learning is pretty broad (and somewhat vague). In this book (and many other books), machine learning refers to “statistical learning” or “data-driven discovery”: finding information from data. Successful machine learning often requires vast amounts of data to learn from. Machine learning discovers patterns in the data and uses the patterns to predict or infer that unseen data has the same (or similar) patterns. For example, a computer program may discover that a person has a high debt-income ratio is likely to default in a mortgage. If a future mortgage applicant has a high debt-income ratio, the program could suggest denying the application due to the higher risk.

As explained earlier, machine learning can be used when many factors need to be considered. Machine learning has already been used in many applications, such as improving customer relationships, making financial decisions, diagnosize illness, identify spam emails, recognize speeches and objects in images.

## Limitations of Machine Learning¶

Machine learning is not perfect; machine learning has some limitations. First, what can be learned depends on the input data. If some important pieces of data are missing (for example, there are no cases of high debt-income ratios), then the computer program cannot learn. To think of this in a different way, a person that grows up inland and has never seen a cargo ship will thus not know existence of cargo ships. Second, it is difficult to determine when the data is sufficient or representative. The patterns are unknown (otherwise, there is no need to learn) so it is hard to tell when there is enough data to discover the patterns. Third, the data may be “biased” and it is not easy to define success. Imagine that you are designing a machine learning program to diagnose a rare illness. If a person has this illness, the program says “Yes”; otherwise, the program says, “No”. Suppose the probability of this illness is one out of 100,000 people. The program would be 99.999% accurate if it always says “No”. However, this high accuracy does not really help. Fourth, each machine learning program reflects a specific machine model that is designed to recognize the patterns in the data. Different models have different capabilities: some models can recognize complex patterns and some others cannot.

## Black Swan Events¶

If an event has never happened, people may think it will never happen. A “black swan event” (The Black Swan by Nassim Nicholas Taleb) is something that has never been seen before and thus considered impossible. People used to believe that swans must be white. Apparently, it is not possible to learn by looking at white swans and infer the existence of a black swan. Black swan events are actually everywhere, if you pay close attention. Before 1969/07/20, nobody could expect that a man would be able to walk on the moon. Before April 2010, nobody would expect a volcano eruption could cause worldwide disruption of air travel. Before the first iPhone was announced, there was no iPhone. Before Michael Phelps won 28 Olympic medals, nobody had won 28 Olympic medals.

If something has been observed, it is definitely possible. After a black swan has been seen, people know that swans can be black. If something has not been observed, it is difficult to say whether it is impossible or not. Maybe it can be observed later.

Even though black swan events cannot be inferred from known patterns, it is possible to predict the occurrence by some people that are willing to challenge these patterns and consider possibilities that have no been observed (i.e., learned). This is where “imagination” and “creativity” come in. Even though machine learning can be very helpful, the current form of machine learning (statistical learning) does not have imagination or creativity.