classification of learning methods...ppt

1
CS 391L: Machine Learning:
Inductive Classification
Raymond J. Mooney
University of Texas at Austin

2
Classification (Categorization)
• Given:
– A description of an instance, xX, where X is the
instance language or instance space.
– A fixed set of categories: C={c1, c2,…cn}
• Determine:
– The category of x: c(x)C, where c(x) is a
categorization function whose domain is X and whose
range is C.
– If c(x) is a binary function C={0,1} ({true,false},
{positive, negative}) then it is called a concept.

3
Learning for Categorization
• A training example is an instance xX,
paired with its correct category c(x):
<x, c(x)> for an unknown categorization
function, c.
• Given a set of training examples, D.
• Find a hypothesized categorization
function, h(x), such that:
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Consistency

4
Sample Category Learning Problem
• Instance language: <size, color, shape>
– size  {small, medium, large}
– color  {red, blue, green}
– shape  {square, circle, triangle}
• C = {positive, negative}
• D: Example Size Color Shape Category
1 small red circle positive
2 large red circle positive
3 small red triangle negative
4 large blue circle negative

5
Hypothesis Selection
• Many hypotheses are usually consistent with the
training data.
– red & circle
– (small & circle) or (large & red)
– (small & red & circle) or (large & red & circle)
– not [ ( red & triangle) or (blue & circle) ]
– not [ ( small & red & triangle) or (large & blue & circle) ]
• Bias
– Any criteria other than consistency with the training data
that is used to select a hypothesis.

6
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.

7
Hypothesis Space
• Restrict learned functions a priori to a given hypothesis
space, H, of functions h(x) that can be considered as
definitions of c(x).
• For learning concepts on instances described by n discrete-
valued features, consider the space of conjunctive
hypotheses represented by a vector of n constraints
<c1, c2, … cn> where each ci is either:
– ?, a wild card indicating no constraint on the ith feature
– A specific value from the domain of the ith feature
– Ø indicating no value is acceptable
• Sample conjunctive hypotheses are
– <big, red, ?>
– <?, ?, ?> (most general hypothesis)
– < Ø, Ø, Ø> (most specific hypothesis)

8
Inductive Learning Hypothesis
• Any function that is found to approximate the target
concept well on a sufficiently large set of training
examples will also approximate the target function well on
unobserved examples.
• Assumes that the training and test examples are drawn
independently from the same underlying distribution.
• This is a fundamentally unprovable hypothesis unless
additional assumptions are made about the target concept
and the notion of “approximating the target function well
on unobserved examples” is defined appropriately (cf.
computational learning theory).

9
Evaluation of Classification Learning
• Classification accuracy (% of instances
classified correctly).
– Measured on an independent test data.
• Training time (efficiency of training
algorithm).
• Testing time (efficiency of subsequent
classification).

10
Category Learning as Search
• Category learning can be viewed as searching the
hypothesis space for one (or more) hypotheses that are
consistent with the training data.
• Consider an instance space consisting of n binary features
which therefore has 2n
instances.
• For conjunctive hypotheses, there are 4 choices for each
feature: Ø, T, F, ?, so there are 4n
syntactically distinct
hypotheses.
• However, all hypotheses with 1 or more Øs are equivalent,
so there are 3n
+1 semantically distinct hypotheses.
• The target binary categorization function in principle could
be any of the possible 22^n
functions on n input bits.
• Therefore, conjunctive hypotheses are a small subset of the
space of possible functions, but both are intractably large.
• All reasonable hypothesis spaces are intractably large or
even infinite.

11
Learning by Enumeration
• For any finite or countably infinite hypothesis
space, one can simply enumerate and test
hypotheses one at a time until a consistent one is
found.
For each h in H do:
If h is consistent with the training data D,
then terminate and return h.
• This algorithm is guaranteed to terminate with a
consistent hypothesis if one exists; however, it is
obviously computationally intractable for almost
any practical problem.

12
Efficient Learning
• Is there a way to learn conjunctive concepts
without enumerating them?
• How do human subjects learn conjunctive
concepts?
• Is there a way to efficiently find an
unconstrained boolean function consistent
with a set of discrete-valued training
instances?
• If so, is it a useful/practical algorithm?

13
Conjunctive Rule Learning
• Conjunctive descriptions are easily learned by finding
all commonalities shared by all positive examples.
• Must check consistency with negative examples. If
inconsistent, no conjunctive rule exists.
Example Size Color Shape Category
Learned rule: red & circle → positive

14
Limitations of Conjunctive Rules
• If a concept does not have a single set of necessary
and sufficient conditions, conjunctive learning
fails.
5 medium red circle negative
Learned rule: red & circle → positive
Inconsistent with negative example #5!

15
Disjunctive Concepts
• Concept may be disjunctive.
Learned rules: small & circle → positive
large & red → positive

16
Using the Generality Structure
• By exploiting the structure imposed by the generality of
hypotheses, an hypothesis space can be searched for
consistent hypotheses without enumerating or explicitly
exploring all hypotheses.
• An instance, xX, is said to satisfy an hypothesis, h, iff
h(x)=1 (positive)
• Given two hypotheses h1 and h2, h1 is more general than or
equal to h2 (h1h2) iff every instance that satisfies h2 also
satisfiesh1.
• Given two hypotheses h1 and h2, h1 is (strictly) more general
than h2 (h1>h2) iff h1h2 and it is not the case that h2 h1.
• Generality defines a partial order on hypotheses.

17
Examples of Generality
• Conjunctive feature vectors
– <?, red, ?> is more general than <?, red, circle>
– Neither of <?, red, ?> and <?, ?, circle> is more general
than the other.
• Axis-parallel rectangles in 2-d space
– A is more general than B
– Neither of A and C are more general than the other.
A
B
C

18
Sample Generalization Lattice
< Ø, Ø, Ø>
<?,?,circ> <big,?,?> <?,red,?> <?,blue,?> <sm,?,?> <?,?,squr>
< big,red,circ><sm,red,circ><big,blue,circ><sm,blue,circ>< big,red,squr><sm,red,squr><big,blue,squr><sm,blue,squr>
< ?,red,circ><big,?,circ><big,red,?><big,blue,?><sm,?,circ><?,blue,circ> <?,red,squr><sm.?,sqr><sm,red,?><sm,blue,?><big,?,squr><?,blue,squr>
<?, ?, ?>
Size: {sm, big} Color: {red, blue} Shape: {circ, squr}
Number of hypotheses = 33
+ 1 = 28

19
Most Specific Learner
(Find-S)
• Find the most-specific hypothesis (least-general generalization,
LGG) that is consistent with the training data.
• Incrementally update hypothesis after every positive example,
generalizing it just enough to satisfy the new example.
• For conjunctive feature vectors, this is easy:
Initialize h = <Ø, Ø,… Ø>
For each positive training instance x in D
For each feature fi
If the constraint on fi in h is not satisfied by x
If fi in h is Ø
then set fi in h to the value of fi in x
else set fi in h to “?”
If h is consistent with the negative training instances in D
then return h
else no consistent hypothesis exists
Time complexity:
O(|D| n)
if n is the number
of features

20
Properties of Find-S
• For conjunctive feature vectors, the most-specific
hypothesis is unique and found by Find-S.
• If the most specific hypothesis is not consistent
with the negative examples, then there is no
consistent function in the hypothesis space, since,
by definition, it cannot be made more specific and
retain consistency with the positive examples.
• For conjunctive feature vectors, if the most-
specific hypothesis is inconsistent, then the target
concept must be disjunctive.

21
Another Hypothesis Language
• Consider the case of two unordered objects each described
by a fixed set of attributes.
– {<big, red, circle>, <small, blue, square>}
• What is the most-specific generalization of:
– Positive: {<big, red, triangle>, <small, blue, circle>}
– Positive: {<big, blue, circle>, <small, red, triangle>}
• LGG is not unique, two incomparable generalizations are:
– {<big, ?, ?>, <small, ?, ?>}
– {<?, red, triangle>, <?, blue, circle>}
• For this space, Find-S would need to maintain a
continually growing set of LGGs and eliminate those that
cover negative examples.
• Find-S is no longer tractable for this space since the
number of LGGs can grow exponentially.

22
Issues with Find-S
• Given sufficient training examples, does Find-S converge
to a correct definition of the target concept (assuming it is
in the hypothesis space)?
• How de we know when the hypothesis has converged to a
correct definition?
• Why prefer the most-specific hypothesis? Are more
general hypotheses consistent? What about the most-
general hypothesis? What about the simplest hypothesis?
• If the LGG is not unique
– Which LGG should be chosen?
– How can a single consistent LGG be efficiently computed or
determined not to exist?
• What if there is noise in the training data and some
training examples are incorrectly labeled?

23
Effect of Noise in Training Data
• Frequently realistic training data is corrupted by errors
(noise) in the features or class values.
• Such noise can result in missing valid generalizations.
– For example, imagine there are many positive examples like #1
and #2, but out of many negative examples, only one like #5 that
actually resulted from a error in labeling.

24
Version Space
• Given an hypothesis space, H, and training
data, D, the version space is the complete
subset of H that is consistent with D.
• The version space can be naively generated
for any finite H by enumerating all
hypotheses and eliminating the inconsistent
ones.
• Can one compute the version space more
efficiently than using enumeration?

25
Version Space with S and G
• The version space can be represented more compactly by
maintaining two boundary sets of hypotheses, S, the set of
most specific consistent hypotheses, and G, the set of most
general consistent hypotheses:
• S and G represent the entire version space via its boundaries in
the generalization lattice:
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26
Version Space Lattice
< Ø, Ø, Ø>
<?,?,circ> <big,?,?> <?,red,?> <?,blue,?> <sm,?,?> <?,?,squr>
< big,red,circ><sm,red,circ><big,blue,circ><sm,blue,circ>< big,red,squr><sm,red,squr><big,blue,squr><sm,blue,squr>
< ?,red,circ><big,?,circ><big,red,?><big,blue,?><sm,?,circ><?,blue,circ> <?,red,squr><sm.?,sqr><sm,red,?><sm,blue,?><big,?,squr><?,blue,squr>
<?, ?, ?>
Size: {sm, big} Color: {red, blue} Shape: {circ, squr}
<<big, red, squr> positive>
<<sm, blue, circ> negative>
Color Code:
G
S
other VS

27
Candidate Elimination (Version Space)
Algorithm
Initialize G to the set of most-general hypotheses in H
Initialize S to the set of most-specific hypotheses in H
For each training example, d, do:
If d is a positive example then:
Remove from G any hypotheses that do not match d
For each hypothesis s in S that does not match d
Remove s from S
Add to S all minimal generalizations, h, of s such that:
1) h matches d
2) some member of G is more general than h
Remove from S any h that is more general than another hypothesis in S
If d is a negative example then:
Remove from S any hypotheses that match d
For each hypothesis g in G that matches d
Remove g from G
Add to G all minimal specializations, h, of g such that:
1) h does not match d
2) some member of S is more specific than h
Remove from G any h that is more specific than another hypothesis in
G

28
Required Subroutines
• To instantiate the algorithm for a specific
hypothesis language requires the following
procedures:
– equal-hypotheses(h1, h2)
– more-general(h1, h2)
– match(h, i)
– initialize-g()
– initialize-s()
– generalize-to(h, i)
– specialize-against(h, i)

29
Minimal Specialization and Generalization
• Procedures generalize-to and specialize-against
are specific to a hypothesis language and can be
complex.
• For conjunctive feature vectors:
– generalize-to: unique, see Find-S
– specialize-against: not unique, can convert each “?” to
an alernative non-matching value for this feature.
• Inputs:
– h = <?, red, ?>
– i = <small, red, triangle>
• Outputs:
– <big, red, ?>
– <medium, red, ?>
– <?, red, square>
– <?, red, circle>

30
Sample VS Trace
S= {< Ø, Ø, Ø>}; G= {<?, ?, ?>}
Positive: <big, red, circle>
Nothing to remove from G
Minimal generalization of only S element is <big, red, circle> which is more
specific than G.
S={<big, red, circle>}; G={<?, ?, ?>}
Negative: <small, red, triangle>
Nothing to remove from S.
Minimal specializations of <?, ?, ?> are <medium, ?, ?>, <big, ?, ?>,
<?, blue, ?>, <?, green, ?>, <?, ?, circle>, <?, ?, square> but most are not
more general than some element of S
S={<big, red, circle>}; G={<big, ?, ?>, <?, ?, circle>}

31
Sample VS Trace (cont)
S={<big, red, circle>}; G={<big, ?, ?>, <?, ?, circle>}
Positive: <small, red, circle>
Remove <big, ?, ?> from G
Minimal generalization of <big, red, circle> is <?, red, circle>
S={<?, red, circle>}; G={<?, ?, circle>}
Negative: <big, blue, circle>
Nothing to remove from S
Minimal specializations of <?, ?, circle> are <small, ? circle>,
<medium, ?, circle>, <?, red, circle>, <?, green, circle> but most are not more
general than some element of S.
S={<?, red, circle>}; G={<?, red, circle>}
S=G; Converged!

32
Properties of VS Algorithm
• S summarizes the relevant information in the positive
examples (relative to H) so that positive examples do not
need to be retained.
• G summarizes the relevant information in the negative
examples, so that negative examples do not need to be
retained.
• Result is not affected by the order in which examples are
processes but computational efficiency may.
• Positive examples move the S boundary up; Negative
examples move the G boundary down.
• If S and G converge to the same hypothesis, then it is the
only one in H that is consistent with the data.
• If S and G become empty (if one does the other must also)
then there is no hypothesis in H consistent with the data.

33
Sample Weka VS Trace 1
java weka.classifiers.vspace.ConjunctiveVersionSpace -t figure.arff -T figure.arff -v -P
Initializing VersionSpace
S = [[#,#,#]]
G = [[?,?,?]]
Instance: big,red,circle,positive
S = [[big,red,circle]]
G = [[?,?,?]]
Instance: small,red,square,negative
G = [[big,?,?], [?,?,circle]]
Instance: small,red,circle,positive
S = [[?,red,circle]]
G = [[?,?,circle]]
Instance: big,blue,circle,negative
G = [[?,red,circle]]
Version Space converged to a single hypothesis.

34
java weka.classifiers.vspace.ConjunctiveVersionSpace -t figure2.arff -T figure2.arff -v -P
S = [[#,#,#]]
G = [[?,?,?]]
G = [[?,?,?]]
Instance: small,blue,triangle,negative
G = [[big,?,?], [?,red,?], [?,?,circle]]
G = [[?,red,?], [?,?,circle]]
Instance: medium,green,square,negative
G = [[?,red,?], [?,?,circle]]

35
S = [[#,#,#]]
G = [[?,?,?]]
G = [[?,?,?]]
Instance: small,red,triangle,negative
G = [[big,?,?], [?,?,circle]]
G = [[?,?,circle]]
Instance: small,green,circle,positive
S = []
G = []
Language is insufficient to describe the concept.

36
S = [[#,#,#]]
G = [[?,?,?]]
Instance: small,red,square,negative
S = [[#,#,#]]
G = [[medium,?,?], [big,?,?], [?,blue,?], [?,green,?], [?,?,circle], [?,?,triangle]]
S = [[#,#,#]]
G = [[medium,?,?], [?,green,?], [?,?,triangle], [big,red,?], [big,?,square], [small,blue,?], [?,blue,square],
[small,?,circle], [?,red,circle]]
G = [[big,red,?], [?,red,circle]]

37
Correctness of Learning
• Since the entire version space is maintained, given
a continuous stream of noise-free training
examples, the VS algorithm will eventually
converge to the correct target concept if it is in the
hypothesis space, H, or eventually correctly
determine that it is not in H.
• Convergence is correctly indicated when S=G.

38
Computational Complexity of VS
• Computing the S set for conjunctive feature
vectors is linear in the number of features and the
number of training examples.
• Computing the G set for conjunctive feature
vectors is exponential in the number of training
examples in the worst case.
• In more expressive languages, both S and G can
grow exponentially.
• The order in which examples are processed can
significantly affect computational complexity.

39
Active Learning
• In active learning, the system is responsible for
selecting good training examples and asking a
teacher (oracle) to provide a class label.
• In sample selection, the system picks good
examples to query by picking them from a
provided pool of unlabeled examples.
• In query generation, the system must generate the
description of an example for which to request a
label.
• Goal is to minimize the number of queries
required to learn an accurate concept description.

40
Active Learning with VS
• An ideal training example would eliminate half of the
hypotheses in the current version space regardless of its label.
• If a training example matches half of the hypotheses in the
version space, then the matching half is eliminated if the
example is negative, and the other (non-matching) half is
eliminated if the example is positive.
• Example:
– Assume training set
• Positive: <big, red, circle>
• Negative: <small, red, triangle>
– Current version space:
• {<big, red, circle>, <big, red, ?>, <big, ?, circle>, <?, red, circle>
<?, ?, circle> <big, ?, ?>}
– An optimal query: <big, blue, circle>
• Given a ceiling of log2|VS| such examples will result in
convergence. This is the best possible guarantee in general.

41
Using an Unconverged VS
• If the VS has not converged, how does it classify a novel
test instance?
• If all elements of S match an instance, then the entire
version space much (since it is more general) and it can be
confidently classified as positive (assuming target concept
is in H).
• If no element of G matches an instance, then the entire
version space must not (since it is more specific) and it can
be confidently classified as negative (assuming target
concept is in H).
• Otherwise, one could vote all of the hypotheses in the VS
(or just the G and S sets to avoid enumerating the VS) to
give a classification with an associated confidence value.
• Voting the entire VS is probabilistically optimal assuming
the target concept is in H and all hypotheses in H are
equally likely a priori.

42
Learning for Multiple Categories
• What if the classification problem is not concept learning
and involves more than two categories?
• Can treat as a series of concept learning problems, where
for each category, Ci, the instances of Ci are treated as
positive and all other instances in categories Cj, ji are
treated as negative (one-versus-all).
• This will assign a unique category to each training instance
but may assign a novel instance to zero or multiple
categories.
• If the binary classifier produces confidence estimates (e.g.
based on voting), then a novel instance can be assigned to
the category with the highest confidence.
• Other approaches exist, such as learning to discriminate all
pairs of categories (all-pairs) and combining decisions
appropriately during test.

43
Inductive Bias
• A hypothesis space that does not include all possible
classification functions on the instance space incorporates
a bias in the type of classifiers it can learn.
• Any means that a learning system uses to choose between
two functions that are both consistent with the training
data is called inductive bias.
• Inductive bias can take two forms:
– Language bias: The language for representing concepts defines a
hypothesis space that does not include all possible functions (e.g.
conjunctive descriptions).
– Search bias: The language is expressive enough to represent all
possible functions (e.g. disjunctive normal form) but the search
algorithm embodies a preference for certain consistent functions
over others (e.g. syntactic simplicity).

44
Unbiased Learning
• For instances described by n features each with m values, there are
mn
instances. If these are to be classified into c categories, then there
are cm^n
possible classification functions.
– For n=10, m=c=2, there are approx. 3.4x1038
possible functions, of
which only 59,049 can be represented as conjunctions (an incredibly
small percentage!)
• However, unbiased learning is futile since if we consider all possible
functions then simply memorizing the data without any real
generalization is as good an option as any.
• Without bias, the version-space is always trivial. The unique most-
specific hypothesis is the disjunction of the positive instances and
the unique most general hypothesis is the negation of the disjunction
of the negative instances:
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45
Futility of Bias-Free Learning
• A learner that makes no a priori assumptions about the
target concept has no rational basis for classifying any
unseen instances.
• Inductive bias can also be defined as the assumptions that,
when combined with the observed training data, logically
entail the subsequent classification of unseen instances.
– Training-data + inductive-bias |― novel-classifications
• The bias of the VS algorithm (assuming it refuses to
classify an instance unless it is classified the same by all
members of the VS), is simply that H contains the target
concept.
• The rote learner, which refuses to classify any instance
unless it has seen it during training, is the least biased.
• Learners can be partially ordered by their amount of bias
– Rote-learner < VS Algorithm < Find-S

46
No Panacea
• No Free Lunch (NFL) Theorem (Wolpert, 1995)
Law of Conservation of Generalization Performance (Schaffer, 1994)
– One can prove that improving generalization performance on unseen data
for some tasks will always decrease performance on other tasks (which
require different labels on the unseen instances).
– Averaged across all possible target functions, no learner generalizes to
unseen data any better than any other learner.
• There does not exist a learning method that is uniformly better than
another for all problems.
• Given any two learning methods A and B and a training set, D, there
always exists a target function for which A generalizes better (or at
least as well) as B.
– Train both methods on D to produce hypotheses hA and hB.
– Construct a target function that labels all unseen instances according to
the predictions of hA.
– Test hA and hB on any unseen test data for this target function and conclude
that hA is better.

47
Logical View of Induction
• Deduction is inferring sound specific conclusions from
general rules (axioms) and specific facts.
• Induction is inferring general rules and theories from
specific empirical data.
• Induction can be viewed as inverse deduction.
– Find a hypothesis h from data D such that
• h  B |― D
where B is optional background knowledge
• Abduction is similar to induction, except it involves
finding a specific hypothesis, h, that best explains a set of
evidence, D, or inferring cause from effect. Typically, in
this case B is quite large compared to induction and h is
smaller and more specific to a particular event.

48
Induction and the Philosophy of Science
• Bacon (1561-1626), Newton (1643-1727) and the sound
deductive derivation of knowledge from data.
• Hume (1711-1776) and the problem of induction.
– Inductive inferences can never be proven and are always subject to
disconfirmation.
• Popper (1902-1994) and falsifiability.
– Inductive hypotheses can only be falsified not proven, so pick
hypotheses that are most subject to being falsified.
• Kuhn (1922-1996) and paradigm shifts.
– Falsification is insufficient, an alternative paradigm must be
available that is clearly elegant and more explanatory must be
available.
• Ptolmaic epicycles and the Copernican revolution
• Orbit of Mercury and general relativity
• Solar neutrino problem and neutrinos with mass
• Postmodernism: Objective truth does not exist; relativism;
science is a social system of beliefs that is no more valid
than others (e.g. religion).

49
Ockham (Occam)’s Razor
• William of Ockham (1295-1349) was a Franciscan
friar who applied the criteria to theology:
– “Entities should not be multiplied beyond necessity”
(Classical version but not an actual quote)
– “The supreme goal of all theory is to make the
irreducible basic elements as simple and as few as
possible without having to surrender the adequate
representation of a single datum of experience.”
(Einstein)
• Requires a precise definition of simplicity.
• Acts as a bias which assumes that nature itself is
simple.
• Role of Occam’s razor in machine learning
remains controversial.

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