1 Machine Learning Introduction Paola Velardi

2 Course material Slides (partly) from: 91L/ 91L/ Textbook: Tom Mitchell, Machine Learning, McGraw Hill, Additional material and papers will be supplied during the last part of the course Course twiki Auto Auto

3 Course Syllabus 1.Concept Learning and the General-to-Specific Ordering 2.Decision Tree Learning 3.Ensamble methods 4.Evaluation methods: experimental and theoretical methods 5.Data mining 6.Artificial Neural Networks 7.Support Vector Machines 8.Clustering 9.Bayesian Learning 10.Q-learning and Genetic Algorithms Except for first lesson, each algorithm is experimented on Weka toolkit Bring your PC with Weka package installed!

4 Exam Written exam on course material Course project using Weka Project is an application of a ML algorithm (from Weka) to a problem tbd (joint projects available with Web&Social) Projects can be carried on by teams of 2 students “homeworks” carried on during second two hours (and at home)

5 ML: definitions and introduction

6 What is Learning? Herbert Simon: “Learning is any process by which a system improves performance from experience.” What is the task? –Classification –Problem solving

7 What is ML? 1.Classification Assign object/event to one of a given finite set of categories. –Medical diagnosis –Credit card applications or transactions –Fraud detection in e-commerce –Worm detection in network packets –Spam filtering in –Recommended articles in a newspaper –Recommended books, movies, music, or jokes –Financial investments –DNA sequences –Spoken words –Handwritten letters –Astronomical images –Tweets Example (medical diagnosis): given a set of categories (disease types) and a laboratory data (blood test etc.), assign a patient to the appropriate category (disease)

Example of classification: healthcare domain 8 Starting from: clinical records, each describing a pregnancy and a delivery. Each record has 215 features. Learn: how to identify potential risk for emergency section Ex: If No previous non-cesarean delivery, and Abnormal 2 nd Trimester Ultrasound, and Malpresentation ad admission Then Probability Emergency C-Section is 0.6

Example of classification: Risk analysis 9

Summary of examples so far Given –A set of examples (they look like records in a database, but not quite the same..) –A set of features for each example (in databases, attribute-value pairs, in ML feature-value pairs) –A “known” category (either binary or discrete-valued) for a subset of examples (named instances) Learn: –A rule to assign a category to “unknown” instances Does this generalize ? –No! (things might be more complex..) 10

Summary so far (2) There is a set of available records for which a classification is known (outcome of a delivery, debtor solvency) (LEARNING EXPERIENCE) Input data are records with features (age, credit period..) and values (boolean, real..) (OBJECT REPRESENTATION) We need to learn a classification function (risk-of- emergency, credit-approval) which can be a boolean a probability... (REPRESENTATION OF TARGET FUNCTION) The learning algorithm output (MODEL) is a rule (TYPE OF LEARNING ALGORITHM IS RULE LEARNING) How do we evaluate how good is our model? (EVALUATION) 11

12 What is ML? 2) Planning/problem solving tasks Performing actions in an environment in order to achieve a goal. –Solving calculus problems –Playing checkers, chess, or backgammon –Driving a car or a jeep –Flying a plane, helicopter, or rocket –Controlling an elevator –Controlling a character in a video game –Controlling a mobile robot

Examples of problem sloving tasks Game playing (e.g. chess) Planning (e.g. robotics, automated driving..) Example: given a representation of an environment (a domestic environmnt) and a command for a robot (e.g. bring the chair from the kitchen to the dining room), find the best plan to execute the command Objective: learning to move and perform actions in a given environment 13

Example 2: Automated Driving and Collision Warning (Stanford Junior) 14 Another nice example is Google’s self-driving car

How does it work? Combination of machine learning and robotics Basically, three key tasks 1) Precise Localization 2) Obstacle Detection 3) Path Planning Reinforcement learning often used (later in this course) + traditional planning algorithms (A*) 15

Summary of examples so far Given a task (grasping objects, recognizing a scene..) Given –examples on how to solve the problem in specific circumstances (often impossible to represent in terms of feature-value instances!) –OR knowledge of a subset of “good” individual actions (e.g. “center occupation is better in chess game”) Learn –The best strategy (plan) to solve the problem Appears to be quite different from previous task (classification) but sometimes is not.. 16

YOUR TURN: CAN YOU LIST EXAMPLES OF PROBLEMS FOR WHICH ML COULD BE USEFUL? 17

18 Why Study Machine Learning? Engineering Better Computing Systems Develop systems that are too difficult/expensive to construct manually because they require specific detailed skills or knowledge tuned to a specific task (knowledge engineering bottleneck). Develop systems that can automatically adapt and customize themselves to individual users. –Personalized news or mail filter –Personalized tutoring Discover new knowledge from large databases (data mining). –Market basket analysis (e.g. diapers and beer) –Medical text mining (e.g. migraines to calcium channel blockers to magnesium) –Twitter mining

19 Why Study Machine Learning? The Time is Ripe Many basic effective and efficient algorithms available. Large amounts of on-line data available. Large amounts of computational resources available.

20 Related Disciplines Artificial Intelligence Data Mining Probability and Statistics Information theory Numerical optimization Computational complexity theory Control theory (adaptive) Psychology (developmental, cognitive) Neurobiology Linguistics Philosophy

Architecture of a learning system 21 Environment/ Experience Learner Knowledge Performance evaluation the algorithm to learn C(x) An hypothesis h(x) for C(x) Training set Test set How good is h(x)?

ML= classification /problem solving The very fist step, given a problem, is to formalise the task: what would we like to learn? Generally speaking, we can formulate a ML problem in this way: 22 Improve on task T, with respect to performance metric P, based on experience E.

23 Defining the Learning Task: examples T: Playing checkers P: Percentage of games won against an arbitrary opponent E: Playing practice games against itself T: Recognizing hand-written words P: Percentage of words correctly classified E: Database of human-labeled images of handwritten words T: Driving on four-lane highways using vision sensors P: Average distance traveled before a human-judged error E: A sequence of images and steering commands recorded while observing a human driver. T: Categorize messages as spam or legitimate. P: Percentage of messages correctly classified. E: Database of s, some with human-given labels Which-one can be considered classification tasks, which-one are problem solving tasks?

Phases/problems in designing a ML algorithm Modeling the domain objects Choosing a learning experience Modeling the target function Defining a Learning Algorithm Evaluation 24

Example : recognizing lions and frogs

Representation: How do we represent our objects? Simple: color! (e.g. a bitmap) Less simple: silhouette 26

From what experience? Supervised learning Unsupervised learning Reinforcement learning 27

Learning paradigms 28

Supervised learning Either an “expert” (e.g. manually classified examples) or some available database of already classified examples 29   lion frog

Unsupervised learning No examples are available. The learner must be able to identify distinguishing features that differentiate the various classes (clustering) 30

Reinforcement learning No examples are available, but some function is provided to associate a reward (or punishment) to a good (bad) move 31 Frog!! WRONG!!! !

Other issues (more in this course) Modeling the target function Learning Algorithm Evaluation E.g. Pr(x=Lion) probabilistic function, defined in [01] E.g. Neural Networks E.g. number of correct classifications/total classifications 32

MACHINE LEARNING AS A PLANNING TASK: PLAY CHECKERS 33

34 Sample Learning Problem We now consider a “machine learning as problem solving” example Learn to play checkers from self-play We will develop an approach analogous to that used in the first machine learning system developed by Arthur Samuels at IBM in 1959.

35 Training Experience Direct experience: Given sample input and output pairs for a useful target function C. –Checker boards labeled with the correct move, e.g. extracted from record of expert play Indirect experience: Given feedback which is not direct I/O pairs for a useful target function. –Potentially arbitrary sequences of game moves and their final game results. Credit/Blame Assignment Problem: How to assign credit/ blame to individual moves given only indirect feedback?

Example sequence 36 More moves here

37 Source of Training Data: possible cases 1.Provided random examples outside of the learner’s control. Negative examples available or only positive? 2.Good training examples selected by a “benevolent teacher.” An experienced player select good and nearly-good moves 3.Learner can query an oracle about the class (good or bad move) of an unlabeled example in the environment.(qao= obtain from some external source the label for an example) 4.Learner can construct an arbitrary example and query an oracle for its label. 5.Learner can design and run experiments directly in the environment without any human guidance.

38 Training vs. Test Distribution Generally assume that the training and test examples are independently drawn from the same overall distribution of data. –IID: Independently and identically distributed –This simply means that we need to learn and test on “equally representative” data: e.g. not good if learn from Mr. X games and test on Mr. Y games, and is not even good if we train and test ONLY on X &Y’s games, there might be other gaming strategies that the learner would never see, and would fail to recognize when the system is in operation

39 Choosing a Target Function What function is to be learned (representing C(x)) and how will it be used by the performance system (ML algorithm choice)? For checkers, assume we are given a function for generating the legal moves for a given board position and want to decide the best move. –Could learn a function: ChooseMove(board, legal-moves) → best-move –Or could learn an evaluation function, V(board) → R, that gives each board position a score for how favorable it is. V can be used to pick a move by applying each legal move, scoring the resulting board position, and choosing the move that results in the highest scoring board position.

40 Ideal Definition of V(b) If b is a final winning board, then V(b) = 100 If b is a final losing board, then V(b) = –100 If b is a final draw board, then V(b) = 0 Otherwise, then V(b) = V(b´), where b´ is the highest scoring final board position that is achieved starting from b and playing optimally until the end of the game (assuming the opponent plays optimally as well).

41 Approximating V(b) Computing V(b) is intractable since it involves searching the complete exponential game tree. Therefore, this definition is said to be non- operational. An operational definition can be computed in reasonable (polynomial) time. Need to learn an operational approximation to the ideal evaluation function.

42 Representing the Target Function Target function can be represented in many ways: lookup table, symbolic rules, numerical function, neural network (a graph). There is a trade-off between the expressiveness of a representation and the ease of learning. The more expressive a representation, the better it will be at approximating an arbitrary function; however, the more examples will be needed to learn an accurate function.

Example (generic) 43 Only two examples are sufficient to perfectly learn a linear function, many examples are needed to approximate a spline

Often the “real” classification function can only be approximated 44 Y=B0+B1X1+B2X2+ ⋯ +BpXp

45 Linear Function for Representing V(b) In checkers, use a linear approximation of the evaluation function. –bp(b): number of black pieces on board b –rp(b): number of red pieces on board b –bk(b): number of black kings on board b –rk(b): number of red kings on board b –bt(b): number of black pieces threatened (i.e. which can be immediately taken by red on its next turn) –rt(b): number of red pieces threatened Learning problem: need to estimate the w i values

Example bp(b): number of black pieces on board b =16 rp(b): number of red pieces on board b = 16 bk(b): number of black kings on board b = 1 rk(b): number of red kings on board b = 1 bt(b): number of black pieces threatened (i.e. which can be immediately taken by red on its next turn) =0 rt(b): number of red pieces threatened = 0 46 “Sicilian Opening”

47 How do we learn? 1.Direct supervision may be available for the target function, for a set of points B in V(b). 1., 100> (win for black) 2.With indirect feedback, training values can be estimated using temporal difference learning (used in reinforcement learning where supervision is delayed reward, more on that, later in this course). 3.Or, we are provided with sequences of chess moves. We don’t know the value of intermediate positions, but we know the value of the final board (-100,+100,0)

48 1. Least Mean Squares (LMS) Algorithm A gradient descent algorithm that incrementally updates the weights of a linear function in an attempt to minimize the mean squared error Initialize the coefficients w i (at random, or w i =1 for all i) Until error >ε: For each training example b in B do : 1) Compute the absolute error : 2) For each board feature, f i, update its weight, w i : for some small constant (learning rate) c (e.g. 0,1)

49 LMS Discussion Intuitively, LMS executes the following rules: –If the output for an example is correct, make no change. –If the output is too high, lower the weights proportional to the values of their corresponding features, so the overall output decreases –If the output is too low, increase the weights proportional to the values of their corresponding features, so the overall output increases. Under the proper weak assumptions, LMS can be proven to eventually converge to a set of weights that minimizes the mean squared error. LMS is a gradient descent algorithm

Example 50 Initialize V^ with w1=w2=..1 Let’s consider a point of the function for which we know the “correct” value of V ex: es: V(b(3,0,1,0,0,0))=100 e=V-V^=100-(1+1×3+1×0+1×1+1×0+1×0+1×0)=95 w1=1+0,1×3×95=29,5 w3=1+0,1×1×95=10,5 After the first iteration the wi values quickly grow, thus reducing the error! Compute error Update w1,w2

LMS (pseudocode) 51

52 2. Temporal Difference Learning We only know the value of final states in a set of game sequences Estimate training values for intermediate (non- terminal) board positions by the estimated value of their successor in an actual game trace. where successor(b) is the next board position where it is the program’s move in actual play. Values towards the end of the game are initially more accurate and continued training slowly “backs up” accurate values to earlier board positions.

Example: a checkmate sequence 53 The intuition is that the “value” of a move depends on what happens after. We know the value of the last move (100, -100 or 0) and from that, we “backtrack” trough the sequence of moves assigning weight The intuition is that the “value” of a move depends on what happens after. We know the value of the last move (100, -100 or 0) and from that, we “backtrack” trough the sequence of moves assigning weight

From CIS 490 / 730: Artificial Intelligence Summary of Design Choices for Learning to Play Checkers Completed Design Determine Type of Training Experience Games against experts Games against self Table of correct moves Determine Target Function Board  valueBoard  move Determine Representation of Learned Function Polynomial Linear function of six features Artificial neural network Determine Learning Algorithm Gradient descent Linear programming

55 Lessons Learned about Learning Learning can be viewed as using direct or indirect experience to approximate a chosen target function. Function approximation can be viewed as a search through a space of hypotheses (representations of functions) for one that best fits a set of training data. Different learning methods assume different hypothesis spaces (representation languages) and/or employ different search techniques.

56 Various Representations for the Objective/target function C Numerical functions –Linear regression –Neural networks –Support vector machines Symbolic functions –Decision trees –Rules in propositional logic –Rules in first-order predicate logic Instance-based functions –Nearest-neighbor –Case-based Probabilistic Graphical Models –Naïve Bayes –Bayesian networks –Hidden-Markov Models (HMMs) –Probabilistic Context Free Grammars (PCFGs) –Markov networks

57 Various Search Algorithms Gradient descent –Perceptron –Backpropagation Dynamic Programming –HMM Learning –PCFG Learning Divide and Conquer –Decision tree induction –Rule learning Evolutionary Computation –Genetic Algorithms (GAs) –Genetic Programming (GP) –Neuro-evolution

58 Evaluation of Learning Systems Experimental –Conduct controlled cross-validation experiments to compare various methods on a variety of benchmark datasets. –Gather data on their performance, e.g. test accuracy, training-time, testing-time. –Analyze differences for statistical significance. Theoretical –Analyze algorithms mathematically and prove theorems about their: Computational complexity Ability to fit training data Sample complexity (number of training examples needed to learn an accurate function)

Summary Machine learning “general” tasks: classification, problem solving Learning paradigms: supervised, unsupervised, reinforcement Sub-problems: –representation: how to represent domain objects and the target function –algorithm selection: how to learn the target function –evaluation: how to test the performance of the learner 59