1 Some rules  No make-up exams ! If you miss with an official excuse, you get average of your scores in the other exams – at most once.  WP only-if you get at least 40% in the exams before you withdraw.  Grades (roughly): D, D+, C, C+, B, B+, A, A , 53-60, 61-65, 66-70, 71-75, 76-80, 81-85, > 85  Attendance: more than 9 absences  DN You get bonus upto 2 marks (to push up grade)  Absences < 4 and well-behaved

2 Some rules  Never believe in anything but "I can!" It always leads to  "I will",  "I did" and  "I'm glad!"

3 Ch 1: Introduction to ML - Outline  What is machine learning?  Why machine learning?  Well-defined learning problem  Example: checkers  Questions that should be asked about ML

4 What is Machine Learning? Definition: 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. Machine learning is the study of how to make computers learn; the goal is to make computers improve their performance through experience.

5 Successful Applications of ML  Learning to recognize spoken words SPHINX (Lee 1989)  Learning to drive an autonomous vehicle ALVINN (Pomerleau 1989)  Learning to classify celestial objects (Fayyad et al 1995)  Learning to play world-class backgammon TD-GAMMON (Tesauro 1992)  Designing the morphology and control structure of electro-mechanical artefacts GOLEM (Lipton, Pollock 2000)

6 Why Machine Learning?  Some tasks cannot be defined well, except by examples (e.g., recognizing people).  Relationships and correlations can be hidden within large amounts of data. Machine Learning/Data Mining may be able to find these relationships.  The amount of knowledge available about certain tasks might be too large for explicit encoding by humans (e.g., medical diagnostic).

7 Why Machine Learning?  Environments change over time.  New knowledge about tasks is constantly being discovered by humans. It may be difficult to continuously re-design systems “by hand”.  Time is right progress in algorithms & theory growing flood of online data computational power available budding industry

8 Why ML – 3 niches  Data mining medical records --> medical knowledge  Self customizing programs learning newsreader  Applications we can’t program autonomous driving speech recognition

9 Multidisciplinary Field MachineLearning Probability & Statistics ComputationalComplexityTheory InformationTheory Philosophy Neurobiology ArtificialIntelligence

10 Learning Problem  Improving with experience at some task task T performance measure P experience E  Example: Handwriting recognition T: recognize & classify handwritten words in images P: % of words correctly classified E: database with words & classifications

11 More examples...  Checkers T: play checkers P: % of games won E: play against self  Robot driving T: drive on a public highway using vision sensors P: average distance traveled before making an error E: sequence of images and steering commands (observed from a human driver)

12 Designing a Learning System: Problem Description – E.g., Checkers  What experience?  What exactly should be learned?  How to represent it?  What algorithm to learn it? 1. Choosing the Training Experience 2. Choosing the Target Function 3. Choosing a Representation for the Target Function 4. Choosing a Function Approximation Algorithm 5. Final Design

13 Type of Training Experience  Direct or indirect? Direct: board state -> correct move Indirect: outcome of a complete game  Move sequences & final outcomes  Credit assignment problem  Thus, more difficult to obtain  What degree of control over examples? Next slide  Is training experience representative of performance goal? Next slide

14 Training experience - control  Degree of control over examples rely on teacher (who selects informative board states & correct moves) ask teacher (proposes difficult board states, ask for move) complete control (play games against itself & check the outcome)  variations: experiment new states or play minor variations of a promising sequence

15 Training experience - training data  How well does it represent the problem?  Is the training experience representative of the task the system will actually have to solve? It is best if it is, but such a situation cannot systematically be achieved! Distribution of examples Same as future test examples? Most theory makes this assumption  Checkers Training playing against itself Performance evaluated playing against world champion

16 Choosing the Target function  Determine type of knowledge to be learned how this will be used  Checkers legal and best moves legal moves easy, best hard large class of tasks are like this

17 Target function  Program choosing the best move ChooseMove: Board -> Move  “improve P in T” reduces to finding a function  choice is a key decision  difficult to learn given (only) indirect examples Alternative: assign a numerical score  V: Board -> R  Assign a numerical score to each board.  Select the best move by evaluating all successor states of legal moves

18 Definitions for V  Final board states V(b) = 100 if winning, -100 if loosing and 0 if draw  Intermediate states? V(b) = V(b’) where b’ is the best final state accessible from b playing optimal game correct but not effectively computable

19 The real target function  Operational V can be used & computed goal: operational description of the ideal target function The ideal target function can often not be learned and must be approximated  Notation ^V: function actually learned V: the ideal target function

20 Choosing a representation for V  Many possibilities collection of rules, neural network, arithmetic function on board features, etc  Usual tradeoff: the more expressive the representation, the more training examples are necessary to choose among the large number of “representable” possibilities

21 Simple representation  Linear function of board features x1: black pieces, x2: red pieces x3: black kings, x4: red kings x5: black threatened by red x6: red threatened by black  ^V  0 +  1 x1 + … +  6 x6 wi: weights to be learned

22 Note  T, P & E are part of the specification  V and ^V are design choices  Here effect of choices is to reduce the learning problem  to finding numbers  0,…,  6

23 Approximation Algorithm  Obtaining training examples Vt(b): training value examples:  Follows procedure deriving from indirect experience weight adjusting procedure to fit ^V to examples

24 Estimating Vt(b)  Game was won/lost does not mean each state was good/bad early play good, late disaster -> loss  Simple approach: Vt(b) = ^V(b’) b’ is the next state where player is allowed to move surprisingly successful intuition: ^V is more accurate at states close game end

25 Adjusting weights  What is best fit to training data?  One common approach: minimize squared error E E = sum (Vt(b) - ^V(b)) 2 several algorithms known  Properties we want Incremental – in refining weights as examples arrive robust to errors in Vt(b)

26 LMS update rule  “Least mean squares”  REPEAT select a random example b compute error(b) = Vt(b) - ^V(b) For each board feature f i, update weight  i   i +  f i error(b)   : learning rate constant, approx. 0.1

27 Notes about LMS  Actually performs stochastic gradient descent search in the weight space to minimize E --- see Ch. 4  Why works no error: no adjusting pos/neg error: weight increased/decr. if a feature f i does not occur, no adjustment to its weight  i is made

28 Final Design  Four distinct modules 1. performance system gives trace for the given board state (using ^V) 2. critic produces examples Vtr(b), from the trace 3. generalizer produces ^V from training data 4. experiment generator generates new problems (initial board state) for ^V Expt. Gen. Perform. system General. Critic

29 Sequence of Design Choices Determine Type of Training Experience Games against experts Games against self Table of correct moves Board  Move Determine Target Function Board  Value Determine Representation of Learned Function polynomial Linear function of six features Artificial neural network Determine Learning Algorithm Gradient descentLinear programming

30 Useful perspective of ML  Search in space of hypotheses Usually a large space All 6-tuples for checkers!  find the one best fitting to examples and prior knowledge  Different spaces depending on the target function and representation  Space concept gives basis to formal analysis – size of the space, number of examples, confidence in the hypothesis…

31 Issues in ML  Algorithms What generalization methods exist? When (if ever) will they converge? Which are best for which types of problems and representations?  Amount of Training Data How much is sufficient? confidence, data & size of space  Reducing problems learning task --> function approximation

32 Issues in ML  Prior knowledge When & how can it help? Helpful even when approximate?  Choosing experiments Are there good strategies? How do choices affect complexity?  Flexible representations Can the learner automatically modify its representation for better performance?