CSE 634
Data Mining Techniques
Mining Association Rules in Large Databases
Prateek Duble
(105301354
)
Course Instructor: Prof. Anita Wasilewska
State University of New York, Stony Brook
State University of New York, Stony Brook
2
References
Data Mining: Concepts & Techniques
by Jiawei Han and
Micheline Kamber
Presentation Slides of
Prof. Anita Wasilewska
Presentation Slides of the Course Book.
“An Effective Hah Based Algorithm for Mining
Association Rules”
(Apriori Algorithm)
by J.S. Park, M.S.
Chen & P.S.Yu , SIGMOD Conference, 1995.
“Mining Frequent Patterns without candidate generation”
(FP

Tree Method)
by J. Han, J. Pei , Y. Yin & R. Mao ,
SIGMOD Conference, 2000.
State University of New York, Stony Brook
3
Overview
Basic Concepts
of Association Rule Mining
The
Apriori
Algorithm (Mining single
dimensional boolean association rules)
Methods to Improve
Apriori’s Efficiency
Frequent

Pattern Growth (
FP

Growth
) Method
From Association Analysis to
Correlation
Analysis
Summary
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4
Basic Concepts of Association Rule
Mining
Association Rule Mining
Finding frequent patterns, associations, correlations,
or causal structures among sets of items or objects in
transaction databases, relational databases, and other
information repositories.
Applications
Basket data analysis, cross

marketing, catalog
design, loss

leader analysis, clustering, classification, etc.
Examples
Rule form: “
Body
Head [support, confidence]”.
buys(x, “diapers”)
buys(x, “beers”) [0.5%, 60%]
major(x, “CS”) ^ takes(x, “DB”)
grade(x, “A”) [1%, 75%]
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Association
Model:
Problem
Statement
I
={i1, i2, ...., in} a set of
items
J
= P(
I
) set of all subsets of the set of items, elements of
J
are
called
itemsets
Transaction
T:
T
is subset of
I
Data Base:
set of transactions
An
association
rule is an implication of the form :
X

> Y,
where
X, Y are
disjoint
subsets of
I
(elements of
J
)
Problem:
Find rules that have support and confidence greater
that user

specified minimum support and minimun
confidence
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Rule Measures: Support & Confidence
Simple Formulas:
Confidence (A
B)
= #tuples containing both A & B / #tuples
containing A = P(BA) = P(A U B ) / P (A)
Support (A
B)
= #tuples containing both A & B/ total number
of tuples = P(A U B)
What do they actually mean ?
Find all the rules
X & Y
Z
with minimum confidence and
support
support,
s
, probability that a transaction contains {X, Y, Z}
confidence,
c
,
conditional probability that a transaction
having {X, Y} also contains
Z
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Support & Confidence : An Example
TransactionID
ItemsBought
2000
A,B,C
1000
A,C
4000
A,D
5000
B,E,F
Let minimum support 50%, and minimum confidence
50%, then we have,
A
C
(50%, 66.6%)
C
A
(50%, 100%)
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Types of Association Rule Mining
Boolean vs. quantitative associations
(Based on the types of values handled)
buys(x, “SQLServer”) ^ income(x, “DMBook”)
buys(x,
“DBMiner”) [0.2%, 60%]
age(x, “30..39”) ^ income(x, “42..48K”)
buys(x, “PC”) [1%,
75%]
Single dimension vs. multiple dimensional associations
(see ex. Above)
Single level vs. multiple

level analysis
What brands of beers are associated with what brands of
diapers?
Various extensions
Correlation, causality analysis
Association does not necessarily imply correlation or
causality
Constraints enforced
E.g., small sales (sum < 100) trigger big buys (sum > 1,000)?
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Overview
Basic Concepts
of Association Rule Mining
The Apriori Algorithm (Mining single
dimensional boolean association rules)
Methods to Improve
Apriori’s Efficiency
Frequent

Pattern Growth (
FP

Growth
) Method
From Association Analysis to
Correlation
Analysis
Summary
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10
The Apriori Algorithm: Basics
The Apriori Algorithm
is an influential algorithm for
mining frequent itemsets for boolean association rules.
Key Concepts :
•
Frequent Itemsets
: The sets of item which has minimum
support (denoted by L
i
for i
th

Itemset).
•
Apriori Property
: Any subset of frequent itemset must
be frequent.
•
Join Operation
: To find L
k
, a set of candidate k

itemsets
is generated by joining L
k

1
with itself.
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The Apriori Algorithm in a Nutshell
Find the
frequent itemsets
: the sets of items that have
minimum support
A subset of a frequent itemset must also be a frequent
itemset
i.e., if {
AB
} is
a frequent itemset, both {
A
} and {
B
}
should be a frequent itemset
Iteratively find frequent itemsets with cardinality from 1
to
k (k

itemset
)
Use the frequent itemsets to generate association rules.
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The Apriori Algorithm : Pseudo code
Join Step
: C
k
is generated by joining L
k

1
with itself
Prune Step
: Any (k

1)

itemset that is not frequent cannot be a
subset of a frequent k

itemset
Pseudo

code
:
C
k
:
Candidate itemset of size k
L
k
:
frequent itemset of size k
L
1
= {frequent items};
for
(
k
= 1;
L
k
!=
;
k
++)
do begin
C
k+1
= candidates generated from
L
k
;
for each
transaction
t
in database do
increment the count of all candidates in
C
k+1
that are contained in
t
L
k+1
= candidates in
C
k+1
with min_support
end
return
k
L
k
;
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The Apriori Algorithm: Example
Consider a database, D , consisting
of 9 transactions.
Suppose min. support count
required is 2 (i.e.
min_sup = 2/9 =
22 %
)
Let
minimum confidence required
is 70%.
We have to first find out the
frequent itemset using Apriori
algorithm.
Then, Association rules will be
generated using min. support &
min. confidence.
TID
List of Items
T100
I1, I2, I5
T100
I2, I4
T100
I2, I3
T100
I1, I2, I4
T100
I1, I3
T100
I2, I3
T100
I1, I3
T100
I1, I2 ,I3, I5
T100
I1, I2, I3
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Step 1
: Generating 1

itemset Frequent Pattern
Itemset
Sup.Count
{I1}
6
{I2}
7
{I3}
6
{I4}
2
{I5}
2
Itemset
Sup.Count
{I1}
6
{I2}
7
{I3}
6
{I4}
2
{I5}
2
•
In the first iteration of the algorithm, each item is a member of the set
of candidate.
•
The set of frequent 1

itemsets, L
1
, consists of the candidate 1

itemsets satisfying minimum support.
Scan D for
count of each
candidate
Compare candidate
support count with
minimum support
count
C
1
L
1
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Step 2
: Generating 2

itemset Frequent Pattern
Itemset
{I1, I2}
{I1, I3}
{I1, I4}
{I1, I5}
{I2, I3}
{I2, I4}
{I2, I5}
{I3, I4}
{I3, I5}
{I4, I5}
Itemset
Sup.
Count
{I1, I2}
4
{I1, I3}
4
{I1, I4}
1
{I1, I5}
2
{I2, I3}
4
{I2, I4}
2
{I2, I5}
2
{I3, I4}
0
{I3, I5}
1
{I4, I5}
0
Itemset
Sup
Count
{I1, I2}
4
{I1, I3}
4
{I1, I5}
2
{I2, I3}
4
{I2, I4}
2
{I2, I5}
2
Generate
C
2
candidates
from L
1
C
2
C
2
L
2
Scan D for
count of
each
candidate
Compare
candidate
support count
with
minimum
support count
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Step 2
: Generating 2

itemset Frequent Pattern [Cont.]
To discover the set of frequent 2

itemsets, L
2
, the
algorithm uses
L
1
Join
L
1
to generate a candidate set of 2

itemsets, C
2
.
Next, the transactions in D are scanned and the support
count for each candidate itemset in C
2
is accumulated (as
shown in the middle table).
The set of frequent 2

itemsets, L
2
, is then determined,
consisting of those candidate 2

itemsets in C
2
having
minimum support.
Note:
We haven’t used Apriori Property yet.
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Step 3
: Generating 3

itemset Frequent Pattern
Itemset
{I1, I2, I3}
{I1, I2, I5}
Itemset
Sup.
Count
{I1, I2, I3}
2
{I1, I2, I5}
2
Itemset
Sup
Count
{I1, I2, I3}
2
{I1, I2, I5}
2
C
3
C
3
L
3
Scan D for
count of
each
candidate
Compare
candidate
support count
with min
support count
Scan D for
count of
each
candidate
•
The generation of the set of candidate 3

itemsets, C
3
, involves
use of
the Apriori Property.
•
In order to find C
3
, we compute
L
2
Join
L
2
.
•
C
3
= L2
Join
L2 = {{I1, I2, I3}, {I1, I2, I5}, {I1, I3, I5}, {I2, I3, I4}, {I2, I3, I5},
{I2, I4, I5}}.
•
Now,
Join step
is complete and
Prune step
will be used to reduce the
size of C
3
.
Prune step helps to avoid heavy computation due to large C
k
.
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Step 3
: Generating 3

itemset Frequent Pattern [Cont.]
Based on the
Apriori property
that all subsets of a frequent itemset must
also be frequent, we can determine that four latter candidates cannot
possibly be frequent. How ?
For example , lets take
{I1, I2, I3}.
The 2

item subsets of it are {I1, I2}, {I1, I3}
& {I2, I3}. Since all 2

item subsets of {I1, I2, I3} are members of L
2
, We will
keep {I1, I2, I3} in C
3
.
Lets take another example of
{I2, I3, I5}
which shows how the pruning is
performed. The 2

item subsets are {I2, I3}, {I2, I5} & {I3,I5}.
BUT, {I3, I5} is not a member of L
2
and hence it is not frequent
violating
Apriori Property
. Thus We will have to remove {I2, I3, I5} from C
3
.
Therefore, C
3
= {{I1, I2, I3}, {I1, I2, I5}} after checking for all members of
result of Join operation
for
Pruning
.
Now, the transactions in D are scanned in order to determine
L
3
, consisting
of those candidates 3

itemsets in C
3
having minimum support.
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Step 4
: Generating 4

itemset Frequent Pattern
The algorithm uses L
3
Join
L
3
to generate a candidate set
of 4

itemsets,
C
4
. Although the join results in {{I1, I2, I3,
I5}}, this itemset is pruned since its subset {{I2, I3, I5}} is
not frequent.
Thus,
C
4
=
φ
, and algorithm terminates,
having found
all of the frequent items.
This completes our Apriori
Algorithm.
What’s Next ?
These frequent itemsets will be used to generate
strong
association rules
( where strong association rules satisfy
both minimum support & minimum confidence).
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Step 5:
Generating Association Rules from Frequent
Itemsets
Procedure:
•
For each frequent itemset
“l”,
generate all nonempty subsets of
l.
•
For every nonempty subset
s
of
l
, output the rule
“s
(l

s)”
if
support_count(l) / support_count(s) >= min_conf
where
min_conf is minimum confidence threshold.
Back To Example:
We had L = {{I1}, {I2}, {I3}, {I4}, {I5}, {I1,I2}, {I1,I3}, {I1,I5}, {I2,I3}, {I2,I4},
{I2,I5}, {I1,I2,I3}, {I1,I2,I5}}.
Lets take
l
= {I1,I2,I5}.
Its all nonempty subsets are {I1,I2}, {I1,I5}, {I2,I5}, {I1}, {I2}, {I5}.
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Step 5:
Generating Association Rules from Frequent
Itemsets [Cont.]
Let
minimum confidence threshold
is , say 70%.
The resulting association rules are shown below,
each listed with its confidence.
R1: I1 ^ I2
I5
•
Confidence = sc{I1,I2,I5}/sc{I1,I2} = 2/4 = 50%
•
R1 is Rejected.
R2: I1 ^ I5
I2
•
Confidence = sc{I1,I2,I5}/sc{I1,I5} = 2/2 = 100%
•
R2 is Selected.
R3: I2 ^ I5
I1
•
Confidence = sc{I1,I2,I5}/sc{I2,I5} = 2/2 = 100%
•
R3 is Selected.
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Step 5:
Generating Association Rules from
Frequent Itemsets [Cont.]
R4: I1
I2 ^ I5
•
Confidence = sc{I1,I2,I5}/sc{I1} = 2/6 = 33%
•
R4 is Rejected.
R5: I2
I1 ^ I5
•
Confidence = sc{I1,I2,I5}/{I2} = 2/7 = 29%
•
R5 is Rejected.
R6: I5
I1 ^ I2
•
Confidence = sc{I1,I2,I5}/ {I5} = 2/2 = 100%
•
R6 is Selected.
In this way, We have found three strong
association rules.
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Overview
Basic Concepts
of Association Rule Mining
The
Apriori
Algorithm (Mining single
dimensional boolean association rules)
Methods to Improve Apriori’s Efficiency
Frequent

Pattern Growth (
FP

Growth
) Method
From Association Analysis to
Correlation
Analysis
Summary
State University of New York, Stony Brook
24
Methods to Improve Apriori’s Efficiency
Hash

based itemset counting
: A
k

itemset whose corresponding
hashing bucket count is below the threshold cannot be frequent.
Transaction reduction
: A transaction that does not contain any
frequent k

itemset is useless in subsequent scans.
Partitioning:
Any itemset that is potentially frequent in DB must be
frequent in at least one of the partitions of DB.
Sampling
: mining on a subset of given data, lower support
threshold + a method to determine the completeness.
Dynamic itemset counting
: add new candidate itemsets only when
all of their subsets are estimated to be frequent.
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Overview
Basic Concepts
of Association Rule Mining
The
Apriori
Algorithm (Mining single
dimensional boolean association rules)
Methods to Improve
Apriori’s Efficiency
Frequent

Pattern Growth (FP

Growth) Method
From Association Analysis to
Correlation
Analysis
Summary
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Mining Frequent Patterns Without Candidate
Generation
Compress a large database into a compact,
Frequent

Pattern tree (FP

tree)
structure
highly condensed
, but complete for frequent pattern
mining
avoid costly database scans
Develop an
efficient
, FP

tree

based frequent pattern
mining method
A divide

and

conquer methodology: decompose
mining tasks into smaller ones
Avoid candidate generation
: sub

database test only!
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FP

Growth Method : An Example
Consider the same previous
example of a database, D ,
consisting of 9 transactions.
Suppose min. support count
required is 2 (i.e.
min_sup =
2/9 = 22 %
)
The first scan of database is
same as Apriori, which derives
the set of 1

itemsets & their
support counts.
The set of frequent items is
sorted in the order of
descending support count.
The resulting set is denoted as
L = {I2:7, I1:6, I3:6, I4:2, I5:2}
TID
List of Items
T100
I1, I2, I5
T100
I2, I4
T100
I2, I3
T100
I1, I2, I4
T100
I1, I3
T100
I2, I3
T100
I1, I3
T100
I1, I2 ,I3, I5
T100
I1, I2, I3
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FP

Growth Method: Construction of FP

Tree
First, create the
root
of the tree, labeled with “
null
”.
Scan the database D a
second time
. (First time we scanned it to
create 1

itemset and then L).
The items in each transaction are processed in L order (i.e. sorted
order).
A branch is created for
each transaction
with items having their
support count separated by colon.
Whenever the same node is encountered in another transaction, we
just
increment
the support count of the common node or Prefix.
To facilitate tree traversal,
an item header table
is built so that each
item points to its occurrences in the tree via a chain of node

links.
Now, The problem of mining frequent patterns in database is
transformed to that of mining the FP

Tree.
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FP

Growth Method: Construction of FP

Tree
An FP

Tree that registers compressed, frequent pattern information
Item
Id
Sup
Count
Node

link
I2
7
I1
6
I3
6
I4
2
I5
2
I2:7
null{}
I1:2
I1:4
I3:2
I4:1
I3:2
I5:1
I5:1
I3:2
I4:1
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Mining the FP

Tree by Creating Conditional
(sub) pattern bases
Steps:
1.
Start from each
frequent length

1 pattern
(as an initial suffix
pattern).
2.
Construct its
conditional pattern base
which consists of the
set of prefix paths in the FP

Tree co

occurring with suffix
pattern.
3.
Then, Construct its
conditional FP

Tree
& perform mining
on such a tree.
4.
The
pattern growth
is achieved by concatenation of the
suffix pattern with the frequent patterns generated from a
conditional FP

Tree.
5.
The union of all frequent patterns (generated by step 4)
gives
the required frequent itemset.
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31
FP

Tree Example Continued
Now, Following the above mentioned steps:
•
Lets start from I5. The I5 is involved in 2 branches namely {I2 I1 I5: 1} and {I2
I1 I3 I5: 1}.
•
Therefore considering I5 as suffix, its 2 corresponding prefix paths would be
{I2 I1: 1} and {I2 I1 I3: 1}, which forms its conditional pattern base.
Item
Conditional pattern
base
Conditional
FP

Tree
Frequent pattern
generated
I5
{(I2 I1: 1),(I2 I1 I3: 1)}
<I2:2 , I1:2>
I2 I5:2, I1 I5:2, I2 I1 I5: 2
I4
{(I2 I1: 1),(I2: 1)}
<I2: 2>
I2 I4: 2
I3
{(I2 I1: 1),(I2: 2), (I1: 2)}
<I2: 4, I1: 2>,<I1:2>
I2 I3:4, I1, I3: 2 , I2 I1 I3: 2
I2
{(I2: 4)}
<I2: 4>
I2 I1: 4
Mining the FP

Tree by creating conditional (sub) pattern bases
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FP

Tree Example Continued
Out of these, Only I1 & I2 is selected in the conditional FP

Tree
because I3 is not satisfying the minimum support count.
For I1 , support count in conditional pattern base = 1 + 1 = 2
For I2 , support count in conditional pattern base = 1 + 1 = 2
For I3, support count in conditional pattern base = 1
Thus support count for I3 is less than required min_sup which is 2
here.
Now , We have conditional FP

Tree with us.
All frequent pattern corresponding to suffix I5 are generated by
considering all possible combinations of I5 and conditional FP

Tree.
The same procedure is applied to suffixes I4, I3 and I1.
Note:
I2 is not taken into consideration for suffix because it doesn’t
have any prefix at all.
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Why Frequent Pattern Growth Fast ?
Performance study shows
FP

growth is an order of magnitude faster than Apriori,
and is also faster than tree

projection
Reasoning
No candidate generation, no candidate test
Use compact data structure
Eliminate repeated database scan
Basic operation is counting and FP

tree building
State University of New York, Stony Brook
34
Overview
Basic Concepts
of Association Rule Mining
The
Apriori
Algorithm (Mining single
dimensional boolean association rules)
Methods to Improve
Apriori’s Efficiency
Frequent

Pattern Growth (
FP

Growth
) Method
From Association Analysis to Correlation
Analysis
Summary
State University of New York, Stony Brook
35
Association & Correlation
As we can see support

confidence framework can be
misleading; it can identify a rule (A=>B) as
interesting (strong) when, in fact the occurrence of A
might not imply the occurrence of B.
Correlation Analysis
provides an alternative
framework for finding interesting relationships, or
to improve understanding of meaning of some
association rules (
a lift of an association rule
).
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Correlation Concepts
Two item sets
A and B are independent
(the
occurrence of A is independent of the occurrence of
item set B) iff
P(A
B) = P(A)
P(B)
Otherwise A and B are dependent and
correlated
The measure of correlation, or
correlation between
A and B
is given by the formula:
Corr(A,B)= P(A U B ) / P(A) . P(B)
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Correlation Concepts [Cont.]
corr(A,B) >1
means that A and B are
positively
correlated
i.e. the occurrence of one implies the
occurrence of the other.
corr(A,B) < 1
means that the occurrence of A is
negatively correlated
with ( or discourages) the
occurrence of B.
corr(A,B) =1
means that A and B are
independent
and there is
no correlation
between them.
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38
Association & Correlation
The correlation formula can be re

written as
Corr(A,B) = P(BA) / P(B)
We already know that
Support(A
B)= P(AUB)
Confidence(A
B)= P(BA)
That means that,
Confidence(A
B)= corr(A,B) P(B)
So correlation, support and confidence are all different, but the
correlation provides an extra information about the association rule
(A
B).
We say that the correlation
corr(A,B) provides the LIFT of the
association rule (A=>B)
, i.e. A is said to increase (or LIFT) the
likelihood of B by the factor of the value returned by the formula for
corr(A,B).
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Correlation Rules
A correlation rule
is a set of items {i
1
, i
2
, ….i
n
}, where the
items occurrences are correlated.
The correlation value
is given by the correlation formula
and we use
Χ square test to determine if correlation is
statistically significant. The Χ square test can also
determine the negative correlation. We can also form
minimal correlated item sets, etc…
Limitations:
Χ square test is less accurate on the data
tables that are sparse and can be misleading for the
contingency tables larger then 2x2
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40
Summary
Association Rule Mining
Finding interesting association or correlation relationships.
Association rules are generated from
frequent itemsets
.
Frequent itemsets are mined using
Apriori algorithm
or
Frequent

Pattern Growth method.
Apriori property
states that all the subsets of frequent itemsets must
also be frequent.
Apriori algorithm uses
frequent itemsets, join & prune methods and
Apriori property
to derive strong association rules.
Frequent

Pattern Growth
method avoids repeated database
scanning of Apriori algorithm.
FP

Growth method is faster than Apriori algorithm
.
Correlation concepts & rules
can be used to further support our
derived association rules.
State University of New York, Stony Brook
41
Questions ?
Thank You !!!
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