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Association Rule Mining

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Mining Association Rules in Large Databases Association rule mining Algorithms Apriori and FP-Growth Max and closed patterns Mining various kinds of association/correlation rules

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Max-patterns & Close-patterns If there are frequent patterns with many items, enumerating all of them is costly. We may be interested in finding the ‘ boundary ’ frequent patterns. Two types …

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Max-patterns Frequent pattern {a 1, …, a 100 } ( ) + ( ) + … + ( ) = = 1.27*10 30 frequent sub-patterns! Max-pattern: frequent patterns without proper frequent super pattern BCDE, ACD are max-patterns BCD is not a max-pattern TidItems 10A,B,C,D,E 20B,C,D,E, 30A,C,D,F Min_sup=2

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

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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 Minimum support = 2 # Closed = 9 # Maximal = 4 Closed and maximal Closed but not maximal

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Maximal vs Closed Itemsets

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MaxMiner: Mining Max-patterns Idea: generate the complete set- enumeration tree one level at a time, while prune if applicable. (ABCD) A (BCD) B (CD) C (D)D () AB (CD)AC (D)AD () BC (D)BD () CD ()ABC (C) ABCD () ABD ()ACD ()BCD ()

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Local Pruning Techniques (e.g. at node A) Check the frequency of ABCD and AB, AC, AD. If ABCD is frequent, prune the whole sub-tree. If AC is NOT frequent, remove C from the parenthesis before expanding. (ABCD) A (BCD) B (CD) C (D)D () AB (CD)AC (D)AD () BC (D)BD () CD ()ABC (C) ABCD () ABD ()ACD ()BCD ()

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Algorithm MaxMiner Initially, generate one node N=, where h(N)= and t(N)={A,B,C,D}. Consider expanding N, If h(N)t(N) is frequent, do not expand N. If for some it(N), h(N){i} is NOT frequent, remove i from t(N) before expanding N. Apply global pruning techniques … (ABCD)

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Global Pruning Technique (across sub-trees) When a max pattern is identified (e.g. ABCD), prune all nodes (e.g. B, C and D) where h(N)t(N) is a sub-set of it (e.g. ABCD). (ABCD) A (BCD) B (CD) C (D)D () AB (CD)AC (D)AD () BC (D)BD () CD ()ABC (C) ABCD () ABD ()ACD ()BCD ()

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Example TidItems 10A,B,C,D,E 20B,C,D,E, 30A,C,D,F (ABCDEF) ItemsFrequency ABCDEF0 A2 B2 C3 D3 E2 F1 Min_sup=2 Max patterns: A (BCDE) B (CDE)C (DE)E ()D (E)

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Example TidItems 10A,B,C,D,E 20B,C,D,E, 30A,C,D,F (ABCDEF) ItemsFrequency ABCDE1 AB1 AC2 AD2 AE1 Min_sup=2 A (BCDE) B (CDE)C (DE)E ()D (E) AC (D)AD () Max patterns: Node A

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Example TidItems 10A,B,C,D,E 20B,C,D,E, 30A,C,D,F (ABCDEF) ItemsFrequency BCDE2 BC BD BE Min_sup=2 A (BCDE) B (CDE)C (DE)E ()D (E) AC (D)AD () Max patterns: BCDE Node B

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Example TidItems 10A,B,C,D,E 20B,C,D,E, 30A,C,D,F (ABCDEF) ItemsFrequency ACD2 Min_sup=2 A (BCDE) B (CDE)C (DE)E ()D (E) AC (D)AD () Max patterns: BCDE ACD Node AC

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Frequent Closed Patterns For frequent itemset X, if there exists no item y s.t. every transaction containing X also contains y, then X is a frequent closed pattern “ ab ” is a frequent closed pattern Concise rep. of freq pats Reduce # of patterns and rules N. Pasquier et al. In ICDT ’ 99 TIDItems 10a, b, c 20a, b, c 30a, b, d 40a, b, d 50e, f Min_sup=2

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Max Pattern vs. Frequent Closed Pattern max pattern closed pattern if itemset X is a max pattern, adding any item to it would not be a frequent pattern; thus there exists no item y s.t. every transaction containing X also contains y. closed pattern max pattern “ ab ” is a closed pattern, but not max TIDItems 10a, b, c 20a, b, c 30a, b, d 40a, b, d 50e, f Min_sup=2

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Mining Frequent Closed Patterns: CLOSET Flist: list of all frequent items in support ascending order Flist: d-a-f-e-c Divide search space Patterns having d Patterns having a but not d, etc. Find frequent closed pattern recursively Among the transactions having d, cfa is frequent closed cfad is a frequent closed pattern J. Pei, J. Han & R. Mao. CLOSET: An Efficient Algorithm for Mining Frequent Closed Itemsets", DMKD'00. TIDItems 10a, c, d, e, f 20a, b, e 30c, e, f 40a, c, d, f 50c, e, f Min_sup=2

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Multiple-Level Association Rules Items often form hierarchy. Items at the lower level are expected to have lower support. Rules regarding itemsets at appropriate levels could be quite useful. A transactional database can be encoded based on dimensions and levels We can explore shared multi- level mining Food bread milk skim Garelick 2% fatwhite wheat Wonder....

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Mining Multi-Level Associations A top_down, progressive deepening approach: First find high-level strong rules: milk bread [20%, 60%]. Then find their lower-level “weaker” rules: 2% fat milk wheat bread [6%, 50%]. Variations at mining multiple-level association rules. Level-crossed association rules: skim milk Wonder wheat bread Association rules with multiple, alternative hierarchies: full fat milk Wonder bread

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Multi-level Association: Uniform Support vs. Reduced Support Uniform Support: the same minimum support for all levels + One minimum support threshold. No need to examine itemsets containing any item whose ancestors do not have minimum support. – Lower level items do not occur as frequently. If support threshold too high miss low level associations too low generate too many high level associations

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Multi-level Association: Uniform Support vs. Reduced Support Reduced Support: reduced minimum support at lower levels There are 4 search strategies: Level-by-level independent Independent search at all levels (no misses) Level-cross filtering by k-itemset Prune a k-pattern if the corresponding k-pattern at the upper level is infrequent Level-cross filtering by single item Prune an item if its parent node is infrequent Controlled level-cross filtering by single item Consider ‘subfrequent’ items that pass a passage threshold

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Uniform Support Multi-level mining with uniform support Milk [support = 10%] full fat Milk [support = 6%] Skim Milk [support = 4%] Level 1 min_sup = 5% Level 2 min_sup = 5% X

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Reduced Support Multi-level mining with reduced support full fat Milk [support = 6%] Skim Milk [support = 4%] Level 1 min_sup = 5% Level 2 min_sup = 3% Milk [support = 10%]

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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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Computing Interestingness Measure Given a rule X Y, information needed to compute rule interestingness can be obtained from a contingency table YY Xf 11 f 10 f 1+ Xf 01 f 00 f o+ f +1 f +0 |T| Contingency table for X Y f 11 : support of X and Y f 10 : support of X and Y f 01 : support of X and Y f 00 : support of X and Y Used to define various measures u support, confidence, lift, Gini, J-measure, etc.

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Drawback of Confidence Coffee Tea15520 Tea Association Rule: Tea Coffee Confidence= P(Coffee|Tea) = 0.75 but P(Coffee) = 0.9 Although confidence is high, rule is misleading P(Coffee|Tea) =

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Statistical Independence Population of 1000 students 600 students know how to swim (S) 700 students know how to bike (B) 420 students know how to swim and bike (S,B) P(SB) = 420/1000 = 0.42 P(S) P(B) = 0.6 0.7 = 0.42 P(SB) = P(S) P(B) => Statistical independence P(SB) > P(S) P(B) => Positively correlated P(SB) Negatively correlated

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Statistical-based Measures Measures that take into account statistical dependence

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Example: Lift/Interest Coffee Tea15520 Tea Association Rule: Tea Coffee Confidence= P(Coffee|Tea) = 0.75 but P(Coffee) = 0.9 Lift = 0.75/0.9= (< 1, therefore is negatively associated)

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Drawback of Lift & Interest YY X100 X YY X900 X Statistical independence: If P(X,Y)=P(X)P(Y) => Lift = 1

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There are lots of measures proposed in the literature Some measures are good for certain applications, but not for others What criteria should we use to determine whether a measure is good or bad? What about Apriori- style support based pruning? How does it affect these measures?

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Properties of A Good Measure Piatetsky-Shapiro: 3 properties a good measure M must satisfy: M(A,B) = 0 if A and B are statistically independent M(A,B) increase monotonically with P(A,B) when P(A) and P(B) remain unchanged M(A,B) decreases monotonically with P(A) [or P(B)] when P(A,B) and P(B) [or P(A)] remain unchanged

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Comparing Different Measures 10 examples of contingency tables: Rankings of contingency tables using various measures:

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Property under Variable Permutation Does M(A,B) = M(B,A)? Symmetric measures: u support, lift, collective strength, cosine, Jaccard, etc Asymmetric measures: u confidence, conviction, Laplace, J-measure, etc

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