ai4.ppt

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Machine Learning
Learning from Observations

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What is Learning?
• Herbert Simon: “Learning is any process
by which a system improves performance
from experience.”
• “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.”
– Tom Mitchell

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Learning
• Learning is essential for unknown environments,
– i.e., when designer lacks omniscience
• Learning is useful as a system construction
method,
– i.e., expose the agent to reality rather than trying to
write it down
• Learning modifies the agent's decision
mechanisms to improve performance

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Machine Learning
• Machine learning: how to acquire a model
on the basis of data / experience
– Learning parameters (e.g. probabilities)
– Learning structure (e.g. BN graphs)
– Learning hidden concepts (e.g. clustering)

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Machine Learning Areas
• Supervised Learning: Data and corresponding labels
are given
• Unsupervised Learning: Only data is given, no labels
provided
• Semi-supervised Learning: Some (if not all) labels are
present
• Reinforcement Learning: An agent interacting with the
world makes observations, takes actions, and is
rewarded or punished; it should learn to choose actions
in such a way as to obtain a lot of reward

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Supervised Learning : Important Concepts
• Data: labeled instances <xi, y>, e.g. emails marked
spam/not spam
– Training Set
– Held-out Set
– Test Set
• Features: attribute-value pairs which characterize each x
• Experimentation cycle
– Learn parameters (e.g. model probabilities) on training set
– (Tune hyper-parameters on held-out set)
– Compute accuracy of test set
– Very important: never “peek” at the test set!
• Evaluation
– Accuracy: fraction of instances predicted correctly
• Overfitting and generalization
– Want a classifier which does well on test data
– Overfitting: fitting the training data very closely, but not
generalizing well

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Example: Spam Filter
Slide from Mackassy

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Example: Digit Recognition
Slide from Mackassy

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Classification Examples
• In classification, we predict labels y (classes) for inputs x
• Examples:
– OCR (input: images, classes: characters)
– Medical diagnosis (input: symptoms, classes: diseases)
– Automatic essay grader (input: document, classes: grades)
– Fraud detection (input: account activity, classes: fraud / no fraud)
– Customer service email routing
– Recommended articles in a newspaper, recommended books
– DNA and protein sequence identification
– Categorization and identification of astronomical images
– Financial investments
– … many more

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Inductive learning
• Simplest form: learn a function from examples
•
• f is the target function
• An example is a pair (x, f(x))
•
• Pure induction task:
– Given a collection of examples of f, return a
function h that approximates f.
– find a hypothesis h, such that h ≈ f, given a training
set of examples
–
• (This is a highly simplified model of real learning:

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Inductive learning method
• Construct/adjust h to agree with f on training set
• (h is consistent if it agrees with f on all examples)
•
• E.g., curve fitting:
•

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•

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•
• Ockham’s razor: prefer the simplest hypothesis
consistent with data

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Generalization
• Hypotheses must generalize to correctly
classify instances not in the training data.
• Simply memorizing training examples is a
consistent hypothesis that does not
generalize.
• Occam’s razor:
– Finding a simple hypothesis helps ensure
generalization.

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Training Error vs Test Error

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Supervised Learning
• Learning a discrete function: Classification
– Boolean classification:
• Each example is classified as true(positive) or
false(negative).
• Learning a continuous function: Regression

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Data Mining: Concepts and Techniques 20
Classification—A Two-Step Process
• Model construction: describing a set of predetermined classes
– Each tuple/sample is assumed to belong to a predefined class,
as determined by the class label
– The set of tuples used for model construction is training set
– The model is represented as classification rules, decision
trees, or mathematical formulae
• Model usage: for classifying future or unknown objects
– Estimate accuracy of the model
• The known label of test sample is compared with the
classified result from the model
• Test set is independent of training set, otherwise over-
fitting will occur
– If the accuracy is acceptable, use the model to classify data
tuples whose class labels are not known

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Illustrating Classification Task
Apply
Model
Induction
Deduction
Learn
Model
Model
Tid Attrib1 Attrib2 Attrib3 Class
1 Yes Large 125K No
2 No Medium 100K No
3 No Small 70K No
4 Yes Medium 120K No
5 No Large 95K Yes
6 No Medium 60K No
7 Yes Large 220K No
8 No Small 85K Yes
9 No Medium 75K No
10 No Small 90K Yes
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Tid Attrib1 Attrib2 Attrib3 Class
11 No Small 55K ?
12 Yes Medium 80K ?
13 Yes Large 110K ?
14 No Small 95K ?
15 No Large 67K ?
10
Test Set
Learning
algorithm
Training Set

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Data Mining: Concepts and Techniques
Issues: Data Preparation
• Data cleaning
– Preprocess data in order to reduce noise and
handle missing values
• Relevance analysis (feature selection)
– Remove the irrelevant or redundant attributes
• Data transformation
– Generalize data to (higher concepts, discretization)
– Normalize attribute values

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Classification Techniques
• Decision Tree based Methods
• Rule-based Methods
• Naïve Bayes and Bayesian Belief
Networks
• Neural Networks
• Support Vector Machines
• and more...

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Learning decision trees
Example Problem: decide whether to wait for a table at a
restaurant, based on the following attributes:
1. Alternate: is there an alternative restaurant nearby?
2. Bar: is there a comfortable bar area to wait in?
3. Fri/Sat: is today Friday or Saturday?
4. Hungry: are we hungry?
5. Patrons: number of people in the restaurant (None, Some, Full)
6. Price: price range ($, $$, $$$)
7. Raining: is it raining outside?
8. Reservation: have we made a reservation?
9. Type: kind of restaurant (French, Italian, Thai, Burger)
10. WaitEstimate: estimated waiting time (0-10, 10-30, 30-60, >60)

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Feature(Attribute)-based representations
• Examples described by feature(attribute) values
– (Boolean, discrete, continuous)
• E.g., situations where I will/won't wait for a table:
• Classification of examples is positive (T) or negative (F)
•

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Decision trees
• One possible representation for hypotheses
• E.g., here is the “true” tree for deciding whether to wait:

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Expressiveness
• Decision trees can express any function of the input attributes.
• E.g., for Boolean functions, truth table row → path to leaf:
• Trivially, there is a consistent decision tree for any training set with
one path to leaf for each example (unless f nondeterministic in x) but
it probably won't generalize to new examples
• Prefer to find more compact decision trees

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Decision tree learning
• Aim: find a small tree consistent with the training examples
• Idea: (recursively) choose "most significant" attribute as root of
(sub)tree

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Decision Tree Construction Algorithm
• Principle
– Basic algorithm (adopted by ID3, C4.5 and CART): a greedy
algorithm
– Tree is constructed in a top-down recursive divide-and-conquer
manner
• Iterations
– At start, all the training tuples are at the root
– Tuples are partitioned recursively based on selected attributes
– Test attributes are selected on the basis of a heuristic or
statistical measure (e.g, information gain)
• Stopping conditions
– All samples for a given node belong to the same class
– There are no remaining attributes for further partitioning –
–
majority voting is employed for classifying the leaf
– There are no samples left

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March 1, 2023 Data Mining: Concepts and Techniques 30
Decision Tree Induction: Training Dataset
age income student credit_rating buys_computer
<=30 high no fair no
<=30 high no excellent no
31…40 high no fair yes
>40 medium no fair yes
>40 low yes fair yes
>40 low yes excellent no
31…40 low yes excellent yes
<=30 medium no fair no
<=30 low yes fair yes
>40 medium yes fair yes
<=30 medium yes excellent yes
31…40 medium no excellent yes
31…40 high yes fair yes
>40 medium no excellent no
This
follows an
example
of
Quinlan’s
ID3
(Playing
Tennis)

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Tree Induction
• Greedy strategy.
– Split the records based on an attribute test
that optimizes certain criterion.
• Issues
– Determine how to split the records
• How to specify the attribute test condition?
• How to determine the best split?
– Determine when to stop splitting

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Choosing an attribute
• Idea: a good attribute splits the examples into subsets
that are (ideally) "all positive" or "all negative"
• Patrons? is a better choice

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How to determine the Best Split
• Greedy approach:
– Nodes with homogeneous class distribution
are preferred
• Need a measure of node impurity:
C0: 5
C1: 5
C0: 9
C1: 1
Non-homogeneous,
High degree of impurity
Homogeneous,
Low degree of impurity

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Measures of Node Impurity
• Information Gain
• Gini Index
• Misclassification error
Choose attributes to split to achieve minimum impurity

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Attribute Selection Measure:
Information Gain (ID3/C4.5)
 Select the attribute with the highest information gain
 Let pi be the probability that an arbitrary tuple in D belongs
to class Ci, estimated by |Ci, D|/|D|
 Expected information (entropy) needed to classify a tuple in
D:
 Information needed (after using A to split D into v partitions)
to classify D:
 Information gained by branching on attribute A
)
(
log
)
( 2
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Info 
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(D)
Info
Info(D)
Gain(A) A



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Information gain
For the training set, p = n = 6, I(6/12, 6/12) = 1 bit
Consider the attributes Patrons and Type (and others too):
Patrons has the highest IG of all attributes and so is chosen by the DTL
algorithm as the root
bits
0
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4
2
,
4
2
(
12
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Type
IG
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Patrons
IG

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Example contd.
• Decision tree learned from the 12 examples:
• Substantially simpler than “true” tree---a more complex
hypothesis isn’t justified by small amount of data

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Measure of Impurity: GINI
(CART, IBM IntelligentMiner)
• Gini Index for a given node t :
(NOTE: p( j | t) is the relative frequency of class j at node t).
– Maximum (1 - 1/nc) when records are equally distributed among
all classes, implying least interesting information
– Minimum (0.0) when all records belong to one class, implying
most interesting information



j
t
j
p
t
GINI 2
)]
|
(
[
1
)
(
C1 0
C2 6
Gini=0.000
C1 2
C2 4
Gini=0.444
C1 3
C2 3
Gini=0.500
C1 1
C2 5
Gini=0.278

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Splitting Based on GINI
• Used in CART, SLIQ, SPRINT.
• When a node p is split into k partitions (children),
the quality of split is computed as,
where, ni = number of records at child i,
n = number of records at node p.



k
i
i
split i
GINI
n
n
GINI
1
)
(

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Comparison of Attribute Selection Methods
• The three measures return good results but
– Information gain:
• biased towards multivalued attributes
– Gain ratio:
• tends to prefer unbalanced splits in which one
partition is much smaller than the others
– Gini index:
• biased to multivalued attributes
• has difficulty when # of classes is large
• tends to favor tests that result in equal-sized
partitions and purity in both partitions

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Example Algorithm: C4.5
• Simple depth-first construction.
• Uses Information Gain
• Sorts Continuous Attributes at each node.
• Needs entire data to fit in memory.
• Unsuitable for Large Datasets.
• You can download the software from
Internet

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Decision Tree Based Classification
• Advantages:
– Easy to construct/implement
– Extremely fast at classifying unknown records
– Models are easy to interpret for small-sized trees
– Accuracy is comparable to other classification
techniques for many simple data sets
– Tree models make no assumptions about the
distribution of the underlying data : nonparametric
– Have a built-in feature selection method that makes
them immune to the presence of useless variables

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Decision Tree Based Classification
• Disadvantages
– Computationally expensive to train
– Some decision trees can be overly complex that
do not generalise the data well.
– Less expressivity: There may be concepts that are
hard to learn with limited decision trees

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Overfitting and Tree Pruning
• Overfitting: An induced tree may overfit the training
data
– Too many branches, some may reflect anomalies due to noise or outliers
– Poor accuracy for unseen samples
• Two approaches to avoid overfitting
– Prepruning: Halt tree construction early—do not split a node if this would
result in the goodness measure falling below a threshold
• Difficult to choose an appropriate threshold
– Postpruning: Remove branches from a “fully grown” tree—get a sequence of
progressively pruned trees
• Use a set of data different from the training data to decide which is the
“best pruned tree”

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Rule-Based Classifier
• Classify records by using a collection of
“if…then…” rules
• Rule: (Condition)  y
– where
• Condition is a conjunctions of attributes
• y is the class label
– LHS: rule antecedent or condition
– RHS: rule consequent
– Examples of classification rules:
• (Blood Type=Warm)  (Lay Eggs=Yes)  Birds
• (Taxable Income < 50K)  (Refund=Yes)  Evade=No

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Rule-based Classifier (Example)
R1: (Give Birth = no)  (Can Fly = yes)  Birds
R2: (Give Birth = no)  (Live in Water = yes)  Fishes
R3: (Give Birth = yes)  (Blood Type = warm)  Mammals
R4: (Give Birth = no)  (Can Fly = no)  Reptiles
R5: (Live in Water = sometimes)  Amphibians
Name Blood Type Give Birth Can Fly Live in Water Class
human warm yes no no mammals
python cold no no no reptiles
salmon cold no no yes fishes
whale warm yes no yes mammals
frog cold no no sometimes amphibians
komodo cold no no no reptiles
bat warm yes yes no mammals
pigeon warm no yes no birds
cat warm yes no no mammals
leopard shark cold yes no yes fishes
turtle cold no no sometimes reptiles
penguin warm no no sometimes birds
porcupine warm yes no no mammals
eel cold no no yes fishes
salamander cold no no sometimes amphibians
gila monster cold no no no reptiles
platypus warm no no no mammals
owl warm no yes no birds
dolphin warm yes no yes mammals
eagle warm no yes no birds

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age?
student? credit rating?
<=30 >40
no yes yes
yes
31..40
fair
excellent
yes
no
• Example: Rule extraction from our buys_computer decision-tree
IF age = young AND student = no THEN buys_computer = no
IF age = young AND student = yes THEN buys_computer = yes
IF age = mid-age THEN buys_computer = yes
IF age = old AND credit_rating = excellent THEN buys_computer = yes
IF age = young AND credit_rating = fair THEN buys_computer = no
Rule Extraction from a Decision Tree
 Rules are easier to understand than large trees
 One rule is created for each path from the
root to a leaf
 Each attribute-value pair along a path forms a
conjunction: the leaf holds the class prediction

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Classification(Sınıflandırma)
• IDEA: Build a model based on past data to
predict the class of the new data
• Given a collection of records (training set )
– Each record contains a set of attributes, one of the attributes is
the class.
• Find a model for class attribute as a function of
the values of other attributes.
• Goal: previously unseen records should be
assigned a class as accurately as possible.

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Hypothesis spaces
How many distinct decision trees with n Boolean attributes?
= number of Boolean functions
= number of distinct truth tables with 2n rows = 22n
• E.g., with 6 Boolean attributes, there are
18,446,744,073,709,551,616 trees
How many purely conjunctive hypotheses (e.g., Hungry  Rain)?
• Each attribute can be in (positive), in (negative), or out
 3n distinct conjunctive hypotheses
• More expressive hypothesis space
– increases chance that target function can be expressed
– increases number of hypotheses consistent with training set
 may get worse predictions

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Using information theory
• To implement Choose-Attribute in the DTL
algorithm
• Information Content (Entropy):
I(P(v1), … , P(vn)) = Σi=1 -P(vi) log2 P(vi)
• For a training set containing p positive examples
and n negative examples:
n
p
n
n
p
n
n
p
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2
2 log
log
)
,
(

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Information gain
• A chosen attribute A divides the training set E into
subsets E1, … , Ev according to their values for A, where
A has v distinct values.
• Information Gain (IG) or reduction in entropy from the
attribute test:
• Choose the attribute with the largest IG

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Performance measurement
• How do we know that h ≈ f ?
1. Use theorems of computational/statistical learning theory
2. Try h on a new test set of examples
(use same distribution over example space as training set)
Learning curve = % correct on test set as a function of training set size

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