Unit 3.pptx

MINING FREQUENT
PATTERNS, ASSOCIATION
RULES

2
What Is Frequent Pattern Analysis?
 Frequent pattern: a pattern (a set of items, subsequences,
substructures, etc.) that occurs frequently in a data set
 First proposed by Agrawal, Imielinski, and Swami [AIS93] in the context
of frequent itemsets and association rule mining
 Motivation: Finding inherent regularities in data
 What products were often purchased together?— Milk and Bread ?!
 What are the subsequent purchases after buying a PC?
 Can we automatically classify web documents?
 Applications
 Basket data analysis, cross-marketing, catalog design, sale campaign
analysis etc

4
Basic Concepts: Frequent Patterns
 itemset: A set of one or more
items
 k-itemset X = {x1, …, xk}
 (absolute) support, or, support
count of X: Frequency or
occurrence of an itemset X
 (relative) support, s, is the
fraction of transactions that
contains X (i.e., the probability
that a transaction contains X)
 An itemset X is frequent if X’s
support is no less than a minsup
threshold
Customer
buys diaper
Customer
buys both
Customer
buys beer
Tid Items bought
10 Coffee, Nuts, Snacks
20 Milk, Coffee, Diaper
30 Beer, Bread
40 Tea, Sugar, Milk
50 Nuts, Coffee, Sugar, Bread, Milk

5
Basic Concepts: Association Rules
 Find all the rules X  Y with
minimum support and confidence
 support, s, probability that a
transaction contains X and Y
 confidence, c, conditional
probability that a transaction
having X also contains Y
Freq. Pat.: Beer:1, Nuts:2, Diaper:1,
Coffee:3,Milk:3 {Coffee, Milk}:2
 Association rules: (many more!)
 milk  Coffee
 Snacks  Beer
Tid Items bought
10 Coffee, Nuts, Snacks
20 Milk, Coffee, Diaper
30 Beer, Bread
40 Tea, Sugar, Milk
50 Nuts, Coffee, Sugar, Bread, Milk

FIND THE SUPPORT AND CONFIDENCE

7
Scalable Frequent Itemset Mining Methods
 Apriori: A Candidate Generation-and-Test Approach
 FPGrowth: A Frequent Pattern-Growth Approach

8
The Downward Closure Property and Scalable
Mining Methods
 The downward closure property of frequent patterns
 Any subset of a frequent itemset must be frequent
 If {Coffee, Milk, Bread} is frequent, so is {Coffee,
Milk}
 i.e., every transaction having {Coffee, Milk, Bread}
also contains {Coffee, Milk}

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Apriori: A Candidate Generation & Test Approach
 Method:
 Initially, scan DB once to get frequent 1-itemset
 Generate length (k+1) candidate itemsets from length k
frequent itemsets
 Test the candidates against DB
 Terminate when no frequent or candidate set can be
generated

10
The Apriori Algorithm—An Example
Database TDB
1st scan
C1
L1
L2
C2 C2
2nd scan
C3 L3
3rd scan
Tid Items
10 A, C, D
20 B, C, E
30 A, B, C, E
40 B, E
Itemset sup
{A} 2
{B} 3
{C} 3
{D} 1
{E} 3
Itemset sup
{A} 2
{B} 3
{C} 3
{E} 3
Itemset
{A, B}
{A, C}
{A, E}
{B, C}
{B, E}
{C, E}
Itemset sup
{A, B} 1
{A, C} 2
{A, E} 1
{B, C} 2
{B, E} 3
{C, E} 2
Itemset sup
{A, C} 2
{B, C} 2
{B, E} 3
{C, E} 2
Itemset
{B, C, E}
Itemset sup
{B, C, E} 2
Supmin = 2

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The Apriori Algorithm (Pseudo-Code)
Ck: Candidate itemset of size k
Lk : frequent itemset of size k
L1 = {frequent items};
for (k = 1; Lk !=; k++) do begin
Ck+1 = candidates generated from Lk;
for each transaction t in database do
increment the count of all candidates in Ck+1 that
are contained in t
Lk+1 = candidates in Ck+1 with min_support
end
return k Lk;

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Implementation of Apriori
 How to generate candidates?
 Step 1: self-joining Lk
 Step 2: pruning
 Example of Candidate-generation
 L3={abc, abd, acd, ace, bcd}
 Self-joining: L3*L3
 abcd from abc and abd
 acde from acd and ace
 Pruning:
 acde is removed because ade is not in L3
 C4 = {abcd}

18
Construct FP-tree from a Transaction Database
{}
f:4 c:1
b:1
p:1
b:1
c:3
a:3
b:1
m:2
p:2 m:1
Header Table
Item frequency head
f 4
c 4
a 3
b 3
m 3
p 3
min_support = 3
TID Items bought (ordered) frequent items
100 {f, a, c, d, g, i, m, p} {f, c, a, m, p}
200 {a, b, c, f, l, m, o} {f, c, a, b, m}
300 {b, f, h, j, o, w} {f, b}
400 {b, c, k, s, p} {c, b, p}
500 {a, f, c, e, l, p, m, n} {f, c, a, m, p}
1. Scan DB once, find
frequent 1-itemset (single
item pattern)
2. Sort frequent items in
frequency descending
order, f-list
3. Scan DB again, construct
FP-tree

21
Find Patterns Having P From P-conditional Database
 Starting at the frequent item header table in the FP-tree
 Traverse the FP-tree by following the link of each frequent item
p
 Accumulate all of transformed prefix paths of item p to form p’s
conditional pattern base
Conditional pattern bases
itemcond. pattern base
c f:3
a fc:3
b fca:1, f:1, c:1
m fca:2, fcab:1
p fcam:2, cb:1
{}
f:4 c:1
b:1
p:1
b:1
c:3
a:3
b:1
m:2
p:2 m:1
Header Table
Item frequency head
f 4
c 4
a 3
b 3
m 3
p 3

Practice Question
Implement and analyse Association rules for the following dataset with
min_support = 60% and confidence =80%
Find all the frequent item sets using Apriori Algorithm. List the strong association
rules.

Generate Fp-Tree to find frequent
itemsets for following dataset
Tid Items
1 A,B,E
2 B,D
3 B,C
4 A,B,D
5 A,C
6 B,C
7 A,C
8 A,B,C,E
9 A,B,C
Conditional
Pattern Base
Conditional FP Tree Frequent Pattern
Generated

24
Benefits of the FP-tree Structure
 Completeness
 Preserve complete information for frequent pattern
mining
 Never break a long pattern of any transaction
 Compactness
 Reduce irrelevant info—infrequent items are gone
 Items in frequency descending order: the more
frequently occurring, the more likely to be shared
 Never be larger than the original database (not count
node-links and the count field)

25
The Frequent Pattern Growth Mining Method
 Idea: Frequent pattern growth
 Recursively grow frequent patterns by pattern and
database partition
 Method
 For each frequent item, construct its conditional
pattern-base, and then its conditional FP-tree
 Repeat the process on each newly created conditional
FP-tree
 Until the resulting FP-tree is empty, or it contains only
one path—single path will generate all the
combinations of its sub-paths, each of which is a
frequent pattern

Supervised vs. Unsupervised Learning
 Supervised learning (classification)
 Supervision: The training data (observations, measurements,
etc.) are accompanied by labels indicating the class of the
observations
 New data is classified based on the training set
 Unsupervised learning (clustering)
 The class labels of training data is unknown
 Given a set of measurements, observations, etc. with the
aim of establishing the existence of classes or clusters in the
data

 Classification
 predicts categorical class labels (discrete or nominal)
 classifies data (constructs a model) based on the training set and
the values (class labels) in a classifying attribute and uses it in
classifying new data
 Numeric Prediction
 models continuous-valued functions, i.e., predicts unknown or
missing values
 Typical applications
 Credit/loan approval:
 Medical diagnosis: if a tumor is cancerous or benign
 Fraud detection: if a transaction is fraudulent
 Web page categorization: which category it is
Prediction Problems: Classification vs. Numeric
Prediction

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 attribute
 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
 Accuracy rate is the percentage of test set samples that are correctly
classified by the model
 Test set is independent of training set (otherwise overfitting)
 If the accuracy is acceptable, use the model to classify new data
 Note: If the test set is used to select models, it is called validation (test) set

Process (1): Model Construction
Training
Data
NAME RANK YEARS TENURED
Mike Assistant Prof 3 no
Mary Assistant Prof 7 yes
Bill Professor 2 yes
Jim Associate Prof 7 yes
Dave Assistant Prof 6 no
Anne Associate Prof 3 no
Classification
Algorithms
IF rank = ‘professor’
OR years > 6
THEN tenured = ‘yes’
Classifier
(Model)

Process (2): Using the Model in Prediction
Classifier
Testing
Data
NAME RANK YEARS TENURED
Tom Assistant Prof 2 no
Merlisa Associate Prof 7 no
George Professor 5 yes
Joseph Assistant Prof 7 yes
Unseen Data
(Jeff, Professor, 4)
Tenured?

Bayesian Classification: Why?
 A statistical classifier: performs probabilistic prediction, i.e.,
predicts class membership probabilities
 Foundation: Based on Bayes’ Theorem.
 Performance: A simple Bayesian classifier, naïve Bayesian
classifier, has comparable performance with decision tree and
selected neural network classifiers
 Incremental: Each training example can incrementally
increase/decrease the probability that a hypothesis is correct
— prior knowledge can be combined with observed data
 Standard: Even when Bayesian methods are computationally
intractable, they can provide a standard of optimal decision
making against which other methods can be measured

BAYES THEOREM
 Conditional Probability= p(A|B)=p(A n B)/p(B)

Bayes’ Theorem: Basics
 Total probability Theorem:
 Bayes’ Theorem:
 Let X be a data sample (“evidence”): class label is unknown
 Let H be a hypothesis that X belongs to class C
 Classification is to determine P(H|X), (i.e., posteriori probability):
the probability that the hypothesis holds given the observed data
sample X
 P(H) (prior probability): the initial probability
 E.g., X will buy computer, regardless of age, income, …
 P(X): probability that sample data is observed
 P(X|H) (likelihood): the probability of observing the sample X, given
that the hypothesis holds
 E.g., Given that X will buy computer, the prob. that X is 31..40,
medium income
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Prediction Based on Bayes’ Theorem
 Given training data X, posteriori probability of a
hypothesis H, P(H|X), follows the Bayes’ theorem
 Informally, this can be viewed as
posteriori = likelihood x prior/evidence
 Predicts X belongs to Ci iff the probability P(Ci|X) is the
highest among all the P(Ck|X) for all the k classes
 Practical difficulty: It requires initial knowledge of many
probabilities, involving significant computational cost
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NAÏVE BAYES EXAMPLE
(OUTLOOK, TEMP)---PLAY(YES/NO)
YES NO P(YES) P(NO)
SUNNY 2 3
OVERCAST 4 0
RAINY 3 2
TOTAL
YES NO P(YES) P(NO)
HOT 2 2
MILD 4 2
COLD 3 1
TOTAL
OUTLOOK
TEMPERATURE
YES 9
NO 5
PLAYS TENNIS YES OR NO

Naïve Bayes Classifier: Training Dataset
Class:
C1:buys_computer = ‘yes’
C2:buys_computer = ‘no’
Data to be classified:
X = (age <=30,
Income = medium,
Student = yes
Credit_rating = Fair)
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

Naïve Bayes Classifier: An Example
 P(Ci): P(buys_computer = “yes”) = 9/14 = 0.643
P(buys_computer = “no”) = 5/14= 0.357
 Compute P(X|Ci) for each class
P(age = “<=30” | buys_computer = “yes”) = 2/9 = 0.222
P(age = “<= 30” | buys_computer = “no”) = 3/5 = 0.6
P(income = “medium” | buys_computer = “yes”) = 4/9 = 0.444
P(income = “medium” | buys_computer = “no”) = 2/5 = 0.4
P(student = “yes” | buys_computer = “yes) = 6/9 = 0.667
P(student = “yes” | buys_computer = “no”) = 1/5 = 0.2
P(credit_rating = “fair” | buys_computer = “yes”) = 6/9 = 0.667
P(credit_rating = “fair” | buys_computer = “no”) = 2/5 = 0.4
 X = (age <= 30 , income = medium, student = yes, credit_rating = fair)
P(X|Ci) : P(X|buys_computer = “yes”) = 0.222 x 0.444 x 0.667 x 0.667 = 0.044
P(X|buys_computer = “no”) = 0.6 x 0.4 x 0.2 x 0.4 = 0.019
P(X|Ci)*P(Ci) : P(X|buys_computer = “yes”) * P(buys_computer = “yes”) = 0.028
P(X|buys_computer = “no”) * P(buys_computer = “no”) = 0.007
Therefore, X belongs to class (“buys_computer = yes”)
age income student
credit_rating
buys_computer

Decision Tree Induction: An Example
age?
overcast
student? credit rating?
<=30 >40
no yes yes
yes
31..40
fair
excellent
yes
no
age income student credit_rating buys_computer
 Training data set: Buys_computer
 Resulting tree:

Algorithm for Decision Tree Induction
 Basic algorithm (a greedy algorithm)
 Tree is constructed in a top-down recursive divide-and-conquer
manner
 At start, all the training examples are at the root
 Attributes are categorical (if continuous-valued, they are
discretized in advance)
 Examples are partitioned recursively based on selected
attributes
 Test attributes are selected on the basis of a heuristic or
statistical measure (e.g., information gain)
 Conditions for stopping partitioning
 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

Brief Review of Entropy

m = 2

Attribute Selection Measure:
Information Gain (ID3)
 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
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