Association%20Analysis:%20Basic%20Concepts%20and%20Algorithms

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Association Analysis Basic Concepts and
Algorithms
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Association Rule Mining

• Given a set of transactions, find rules that will
  predict the occurrence of an item based on the
  occurrences of other items in the transaction

Example of Association Rules
Market-Basket transactions
Diaper ? Beer,Beer, Bread ? Milk,
Implication means co-occurrence, not causality!
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Definition Frequent Itemset

• Itemset
• A collection of one or more items
• Example Milk, Bread, Diaper
• k-itemset
• An itemset that contains k items
• Support count (?)
• Frequency of occurrence of an itemset
• E.g. ?(Milk, Bread,Diaper) 2
• Support
• Fraction of transactions that contain an itemset
• E.g. s(Milk, Bread, Diaper) 2/5
• Frequent Itemset
• An itemset whose support is greater than or equal
  to a minsup threshold

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Definition Association Rule

• Association Rule
• An implication expression of the form X ? Y,
  where X and Y are itemsets
• Example Milk, Diaper ? Beer
• Rule Evaluation Metrics
• Support (s)
• Fraction of transactions that contain both X and
  Y
• Confidence (c)
• Measures how often items in Y appear in
  transactions thatcontain X

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Association Rule Mining Task

• Given a set of transactions T, the goal of
  association rule mining is to find all rules
  having
• support minsup threshold
• confidence minconf threshold
• Brute-force approach
• List all possible association rules
• Compute the support and confidence for each rule
• Prune rules that fail the minsup and minconf
  thresholds
• ? Computationally prohibitive!

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Mining Association Rules
Example of Rules Milk,Diaper ? Beer (s0.4,
c0.67)Milk,Beer ? Diaper (s0.4,
c1.0) Diaper,Beer ? Milk (s0.4,
c0.67) Beer ? Milk,Diaper (s0.4, c0.67)
Diaper ? Milk,Beer (s0.4, c0.5) Milk ?
Diaper,Beer (s0.4, c0.5)

• Observations
• All the above rules are binary partitions of the
  same itemset Milk, Diaper, Beer
• Rules originating from the same itemset have
  identical support but can have different
  confidence
• Thus, we may decouple the support and confidence
  requirements

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Mining Association Rules

• Two-step approach
• Frequent Itemset Generation
• Generate all itemsets whose support ? minsup
• Rule Generation
• Generate high confidence rules from each frequent
  itemset, where each rule is a binary partitioning
  of a frequent itemset
• Frequent itemset generation is still
  computationally expensive

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Frequent Itemset Generation
Given d items, there are 2d possible candidate
itemsets
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Frequent Itemset Generation

• Brute-force approach
• Each itemset in the lattice is a candidate
  frequent itemset
• Count the support of each candidate by scanning
  the database
• Match each transaction against every candidate
• Complexity O(NMw) gt Expensive since M 2d !!!

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Computational Complexity

• Given d unique items
• Total number of itemsets 2d
• Total number of possible association rules

If d6, R 602 rules
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Frequent Itemset Generation Strategies

• Reduce the number of candidates (M)
• Complete search M2d
• Use pruning techniques to reduce M
• Reduce the number of transactions (N)
• Reduce size of N as the size of itemset increases
• Used by DHP and vertical-based mining algorithms
• Reduce the number of comparisons (NM)
• Use efficient data structures to store the
  candidates or transactions
• No need to match every candidate against every
  transaction

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Reducing Number of Candidates

• Apriori principle
• If an itemset is frequent, then all of its
  subsets must also be frequent
• Apriori principle holds due to the following
  property of the support measure
• Support of an itemset never exceeds the support
  of its subsets
• This is known as the anti-monotone property of
  support

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Illustrating Apriori Principle
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Illustrating Apriori Principle
Items (1-itemsets)
Pairs (2-itemsets) (No need to
generatecandidates involving Cokeor Eggs)
Minimum Support 3
Triplets (3-itemsets)
If every subset is considered, 6C1 6C2 6C3
41 With support-based pruning, 6 6 1 13
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Apriori Algorithm

• Method
• Let k1
• Generate frequent itemsets of length 1
• Repeat until no new frequent itemsets are
  identified
• Generate length (k1) candidate itemsets from
  length k frequent itemsets
• Prune candidate itemsets containing subsets of
  length k that are infrequent
• Count the support of each candidate by scanning
  the DB
• Eliminate candidates that are infrequent, leaving
  only those that are frequent

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Reducing Number of Comparisons

• Candidate counting
• Scan the database of transactions to determine
  the support of each candidate itemset
• To reduce the number of comparisons, store the
  candidates in a hash structure
• Instead of matching each transaction against
  every candidate, match it against candidates
  contained in the hashed buckets

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Review

• What are the association rules?
• What are the frequent itemsets?
• What are support/supporting count?
• Whats apriori principle?
• How to mine FIM?

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Generate Hash Tree

• Suppose you have 15 candidate itemsets of length
  3
• 1 4 5, 1 2 4, 4 5 7, 1 2 5, 4 5 8, 1 5
  9, 1 3 6, 2 3 4, 5 6 7, 3 4 5, 3 5 6,
  3 5 7, 6 8 9, 3 6 7, 3 6 8
• You need
• Hash function
• Max leaf size max number of itemsets stored in
  a leaf node (if number of candidate itemsets
  exceeds max leaf size, split the node)

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Association Rule Discovery Hash tree
Hash Function
Candidate Hash Tree
1,4,7
3,6,9
2,5,8
Hash on 1, 4 or 7
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Association Rule Discovery Hash tree
Hash Function
Candidate Hash Tree
1,4,7
3,6,9
2,5,8
Hash on 2, 5 or 8
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Association Rule Discovery Hash tree
Hash Function
Candidate Hash Tree
1,4,7
3,6,9
2,5,8
Hash on 3, 6 or 9
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Subset Operation
Given a transaction t, what are the possible
subsets of size 3?
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Subset Operation Using Hash Tree
transaction
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Subset Operation Using Hash Tree
transaction
1 3 6
3 4 5
1 5 9
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Subset Operation Using Hash Tree
transaction
1 3 6
3 4 5
1 5 9
Match transaction against 11 out of 15 candidates
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How to Generate Candidates?

• Suppose the items in Lk-1 are listed in an order
• Step 1 self-joining Lk-1
• insert into Ck
• select p.item1, p.item2, , p.itemk-1, q.itemk-1
• from Lk-1 p, Lk-1 q
• where p.item1q.item1, , p.itemk-2q.itemk-2,
  p.itemk-1 lt q.itemk-1
• Step 2 pruning
• forall itemsets c in Ck do
• forall (k-1)-subsets s of c do
• if (s is not in Lk-1) then delete c from Ck

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Challenges of Frequent Pattern Mining

• Challenges
• Multiple scans of transaction database
• Huge number of candidates
• Tedious workload of support counting for
  candidates
• Improving Apriori general ideas
• Reduce passes of transaction database scans
• Shrink number of candidates
• Facilitate support counting of candidates

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Partition Scan Database Only Twice

• Any itemset that is potentially frequent in DB
  must be frequent in at least one of the
  partitions of DB
• Scan 1 partition database and find local
  frequent patterns
• Scan 2 consolidate global frequent patterns
• A. Savasere, E. Omiecinski, and S. Navathe. An
  efficient algorithm for mining association in
  large databases. In VLDB95

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DHP Reduce the Number of Candidates

• A k-itemset whose corresponding hashing bucket
  count is below the threshold cannot be frequent
• Candidates a, b, c, d, e
• Hash entries ab, ad, ae bd, be, de
• Frequent 1-itemset a, b, d, e
• ab is not a candidate 2-itemset if the sum of
  count of ab, ad, ae is below support threshold
• J. Park, M. Chen, and P. Yu. An effective
  hash-based algorithm for mining association
  rules. In SIGMOD95

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Sampling for Frequent Patterns

• Select a sample of original database, mine
  frequent patterns within sample using Apriori
• Scan database once to verify frequent itemsets
  found in sample, only borders of closure of
  frequent patterns are checked
• Example check abcd instead of ab, ac, , etc.
• Scan database again to find missed frequent
  patterns
• H. Toivonen. Sampling large databases for
  association rules. In VLDB96

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DIC Reduce Number of Scans
ABCD

• Once both A and D are determined frequent, the
  counting of AD begins
• Once all length-2 subsets of BCD are determined
  frequent, the counting of BCD begins

ABC
ABD
ACD
BCD
AB
AC
BC
AD
BD
CD
Transactions
1-itemsets
B
C
D
A
2-itemsets
Apriori


Itemset lattice
1-itemsets
2-items
S. Brin R. Motwani, J. Ullman, and S. Tsur.
Dynamic itemset counting and implication rules
for market basket data. In SIGMOD97
3-items
DIC
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Factors Affecting Complexity

• Choice of minimum support threshold
• lowering support threshold results in more
  frequent itemsets
• this may increase number of candidates and max
  length of frequent itemsets
• Dimensionality (number of items) of the data set
• more space is needed to store support count of
  each item
• if number of frequent items also increases, both
  computation and I/O costs may also increase
• Size of database
• since Apriori makes multiple passes, run time of
  algorithm may increase with number of
  transactions
• Average transaction width
• transaction width increases with denser data
  sets
• This may increase max length of frequent itemsets
  and traversals of hash tree (number of subsets in
  a transaction increases with its width)

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Compact Representation of Frequent Itemsets

• Some itemsets are redundant because they have
  identical support as their supersets
• Number of frequent itemsets
• Need a compact representation

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Maximal Frequent Itemset
An itemset is maximal frequent if none of its
immediate supersets is frequent
Maximal Itemsets
Border
Infrequent Itemsets
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Closed Itemset

• An itemset is closed if none of its immediate
  supersets has the same support as the itemset

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Maximal vs Closed Itemsets
Transaction Ids
Not supported by any transactions
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Maximal vs Closed Frequent Itemsets
Closed but not maximal
Minimum support 2
Closed and maximal
Closed 9 Maximal 4
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Maximal vs Closed Itemsets
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Alternative Methods for Frequent Itemset
Generation

• Traversal of Itemset Lattice
• General-to-specific vs Specific-to-general

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Alternative Methods for Frequent Itemset
Generation

• Traversal of Itemset Lattice
• Equivalent Classes

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Alternative Methods for Frequent Itemset
Generation

• Traversal of Itemset Lattice
• Breadth-first vs Depth-first

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Alternative Methods for Frequent Itemset
Generation

• Representation of Database
• horizontal vs vertical data layout

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ECLAT

• For each item, store a list of transaction ids
  (tids)

TID-list
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ECLAT

• Determine support of any k-itemset by
  intersecting tid-lists of two of its (k-1)
  subsets.
• 3 traversal approaches
• top-down, bottom-up and hybrid
• Advantage very fast support counting
• Disadvantage intermediate tid-lists may become
  too large for memory

?
?
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FP-growth Algorithm

• Use a compressed representation of the database
  using an FP-tree
• Once an FP-tree has been constructed, it uses a
  recursive divide-and-conquer approach to mine the
  frequent itemsets

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FP-tree construction
null
After reading TID1
A1
B1
After reading TID2
null
B1
A1
B1
C1
D1
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FP-Tree Construction
Transaction Database
null
B3
A7
B5
C3
C1
D1
D1
Header table
C3
E1
D1
E1
D1
E1
D1
Pointers are used to assist frequent itemset
generation
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FP-growth
Conditional Pattern base for D P
(A1,B1,C1), (A1,B1),
(A1,C1), (A1),
(B1,C1) Recursively apply FP-growth on
P Frequent Itemsets found (with sup gt 1) AD,
BD, CD, ACD, BCD
null
A7
B1
B5
C1
C1
D1
D1
C3
D1
D1
D1
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Rule Generation

• Given a frequent itemset L, find all non-empty
  subsets f ? L such that f ? L f satisfies the
  minimum confidence requirement
• If A,B,C,D is a frequent itemset, candidate
  rules
• ABC ?D, ABD ?C, ACD ?B, BCD ?A, A ?BCD, B
  ?ACD, C ?ABD, D ?ABCAB ?CD, AC ? BD, AD ? BC,
  BC ?AD, BD ?AC, CD ?AB,
• If L k, then there are 2k 2 candidate
  association rules (ignoring L ? ? and ? ? L)

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Rule Generation

• How to efficiently generate rules from frequent
  itemsets?
• In general, confidence does not have an
  anti-monotone property
• c(ABC ?D) can be larger or smaller than c(AB ?D)
• But confidence of rules generated from the same
  itemset has an anti-monotone property
• e.g., L A,B,C,D c(ABC ? D) ? c(AB ? CD)
  ? c(A ? BCD)
• Confidence is anti-monotone w.r.t. number of
  items on the RHS of the rule

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Rule Generation for Apriori Algorithm
Lattice of rules
Low Confidence Rule
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Rule Generation for Apriori Algorithm

• Candidate rule is generated by merging two rules
  that share the same prefixin the rule consequent
• join(CDgtAB,BDgtAC)would produce the
  candidaterule D gt ABC
• Prune rule DgtABC if itssubset ADgtBC does not
  havehigh confidence

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Effect of Support Distribution

• Many real data sets have skewed support
  distribution

Support distribution of a retail data set
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Effect of Support Distribution

• How to set the appropriate minsup threshold?
• If minsup is set too high, we could miss itemsets
  involving interesting rare items (e.g., expensive
  products)
• If minsup is set too low, it is computationally
  expensive and the number of itemsets is very
  large
• Using a single minimum support threshold may not
  be effective

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Multiple Minimum Support

• How to apply multiple minimum supports?
• MS(i) minimum support for item i
• e.g. MS(Milk)5, MS(Coke) 3,
  MS(Broccoli)0.1, MS(Salmon)0.5
• MS(Milk, Broccoli) min (MS(Milk),
  MS(Broccoli)) 0.1
• Challenge Support is no longer anti-monotone
• Suppose Support(Milk, Coke) 1.5
  and Support(Milk, Coke, Broccoli) 0.5
• Milk,Coke is infrequent but Milk,Coke,Broccoli
  is frequent

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Multiple Minimum Support
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Multiple Minimum Support
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Multiple Minimum Support (Liu 1999)

• Order the items according to their minimum
  support (in ascending order)
• e.g. MS(Milk)5, MS(Coke) 3,
  MS(Broccoli)0.1, MS(Salmon)0.5
• Ordering Broccoli, Salmon, Coke, Milk
• Need to modify Apriori such that
• L1 set of frequent items
• F1 set of items whose support is ?
  MS(1) where MS(1) is mini( MS(i) )
• C2 candidate itemsets of size 2 is generated
  from F1 instead of L1

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Multiple Minimum Support (Liu 1999)

• Modifications to Apriori
• In traditional Apriori,
• A candidate (k1)-itemset is generated by
  merging two frequent itemsets of size k
• The candidate is pruned if it contains any
  infrequent subsets of size k
• Pruning step has to be modified
• Prune only if subset contains the first item
• e.g. CandidateBroccoli, Coke, Milk
  (ordered according to minimum support)
• Broccoli, Coke and Broccoli, Milk are
  frequent but Coke, Milk is infrequent
• Candidate is not pruned because Coke,Milk does
  not contain the first item, i.e., Broccoli.

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Mining Various Kinds of Association Rules

• Mining multilevel association
• Miming multidimensional association
• Mining quantitative association
• Mining interesting correlation patterns

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Mining Multiple-Level Association Rules

• Items often form hierarchies
• Flexible support settings
• Items at the lower level are expected to have
  lower support
• Exploration of shared multi-level mining (Agrawal
  Srikant_at_VLB95, Han Fu_at_VLDB95)

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Multi-level Association Redundancy Filtering

• Some rules may be redundant due to ancestor
  relationships between items.
• Example
• milk ? wheat bread support 8, confidence
  70
• 2 milk ? wheat bread support 2, confidence
  72
• We say the first rule is an ancestor of the
  second rule.
• A rule is redundant if its support is close to
  the expected value, based on the rules
  ancestor.

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Mining Multi-Dimensional Association

• Single-dimensional rules
• buys(X, milk) ? buys(X, bread)
• Multi-dimensional rules ? 2 dimensions or
  predicates
• Inter-dimension assoc. rules (no repeated
  predicates)
• age(X,19-25) ? occupation(X,student) ?
  buys(X, coke)
• hybrid-dimension assoc. rules (repeated
  predicates)
• age(X,19-25) ? buys(X, popcorn) ? buys(X,
  coke)
• Categorical Attributes finite number of possible
  values, no ordering among valuesdata cube
  approach
• Quantitative Attributes numeric, implicit
  ordering among valuesdiscretization, clustering,
  and gradient approaches

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Mining Quantitative Associations

• Techniques can be categorized by how numerical
  attributes, such as age or salary are treated
• Static discretization based on predefined concept
  hierarchies (data cube methods)
• Dynamic discretization based on data distribution
  (quantitative rules, e.g., Agrawal
  Srikant_at_SIGMOD96)
• Clustering Distance-based association (e.g.,
  Yang Miller_at_SIGMOD97)
• one dimensional clustering then association
• Deviation (such as Aumann and Lindell_at_KDD99)
• Sex female gt Wage mean7/hr (overall mean
  9)

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Quantitative Association Rules

• Proposed by Lent, Swami and Widom ICDE97
• Numeric attributes are dynamically discretized
• Such that the confidence or compactness of the
  rules mined is maximized
• 2-D quantitative association rules Aquan1 ?
  Aquan2 ? Acat
• Cluster adjacent
  association rules
  to form
  general
  rules using a 2-D grid
• Example

age(X,34-35) ? income(X,30-50K) ?
buys(X,high resolution TV)
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Mining Other Interesting Patterns

• Flexible support constraints (Wang et al. _at_
  VLDB02)
• Some items (e.g., diamond) may occur rarely but
  are valuable
• Customized supmin specification and application
• Top-K closed frequent patterns (Han, et al. _at_
  ICDM02)
• Hard to specify supmin, but top-k with lengthmin
  is more desirable
• Dynamically raise supmin in FP-tree construction
  and mining, and select most promising path to mine

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Pattern Evaluation

• Association rule algorithms tend to produce too
  many rules
• many of them are uninteresting or redundant
• Redundant if A,B,C ? D and A,B ? D
  have same support confidence
• Interestingness measures can be used to
  prune/rank the derived patterns
• In the original formulation of association rules,
  support confidence are the only measures used

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Interestingness Measure Correlations (Lift)

• play basketball ? eat cereal 40, 66.7 is
  misleading
• The overall of students eating cereal is 75 gt
  66.7.
• play basketball ? not eat cereal 20, 33.3 is
  more accurate, although with lower support and
  confidence
• Measure of dependent/correlated events lift

Basketball Not basketball Sum (row)
Cereal 2000 1750 3750
Not cereal 1000 250 1250
Sum(col.) 3000 2000 5000
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Are lift and ?2 Good Measures of Correlation?

• Buy walnuts ? buy milk 1, 80 is
  misleading
• if 85 of customers buy milk
• Support and confidence are not good to represent
  correlations
• So many interestingness measures? (Tan, Kumar,
  Sritastava _at_KDD02)

Milk No Milk Sum (row)
Coffee m, c m, c c
No Coffee m, c m, c c
Sum(col.) m m ?
DB m, c m, c mc mc lift all-conf coh ?2
A1 1000 100 100 10,000 9.26 0.91 0.83 9055
A2 100 1000 1000 100,000 8.44 0.09 0.05 670
A3 1000 100 10000 100,000 9.18 0.09 0.09 8172
A4 1000 1000 1000 1000 1 0.5 0.33 0
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Which Measures Should Be Used?

• lift and ?2 are not good measures for
  correlations in large transactional DBs
• all-conf or coherence could be good measures
  (Omiecinski_at_TKDE03)
• Both all-conf and coherence have the downward
  closure property
• Efficient algorithms can be derived for mining
  (Lee et al. _at_ICDM03sub)