1 Association Rule Mining (II) Instructor: Qiang Yang Thanks: J.Han and J. Pei

Frequent-pattern mining methods2 Bottleneck of Frequent-pattern Mining Multiple database scans are costly Mining long patterns needs many passes of scanning and generates lots of candidates To find frequent itemset i 1 i 2 …i 100 # of scans: 100 # of Candidates: ( ) + ( ) + … + ( ) = = 1.27*10 30 ! Bottleneck: candidate-generation-and-test Can we avoid candidate generation?

Frequent-pattern mining methods3 FP-growth: Frequent-pattern Mining Without Candidate Generation Heuristic: let P be a frequent itemset, S be the set of transactions contain P, and x be an item. If x is a frequent item in S, {x}  P must be a frequent itemset No candidate generation! A compact data structure, FP-tree, to store information for frequent pattern mining Recursive mining algorithm for mining complete set of frequent patterns

Frequent-pattern mining methods4 Example Items Bought f,a,c,d,g,i,m,p a,b,c,f,l,m,o b,f,h,j,o b,c,k,s,p a,f,c,e,l,p,m,n Min Support = 3

Frequent-pattern mining methods5 Scan the database List of frequent items, sorted: (item:support) The root of the tree is created and labeled with “{}” Scan the database Scanning the first transaction leads to the first branch of the tree: Order according to frequency

Frequent-pattern mining methods6 Scanning TID=100 Transaction Database TIDItems 100f,a,c,d,g,i,m,p {} f:1 c:1 a:1 m:1 p:1 Header Table Node Itemcounthead f1 c1 a1 m1 p1 root

Frequent-pattern mining methods7 Scanning TID=200 Frequent Single Items: F1= TID=200 Possible frequent items: Intersect with F1: f,c,a,b,m Along the first branch of, intersect: Generate two children, Items Bought f,a,c,d,g,i,m,p a,b,c,f,l,m,o b,f,h,j,o b,c,k,s,p a,f,c,e,l,p,m,n

Frequent-pattern mining methods8 Scanning TID=200 Transaction Database TIDItems 200f,c,a,b,m {} f:2 c:2 a:2 m:1 p:1 Header Table Node Itemcounthead f1 c1 a1 b1 m2 p1 root b:1 m:1

Frequent-pattern mining methods9 The final FP-tree Transaction Database TIDItems 100f,a,c,d,g,i,m,p 200a,b,c,f,l,m,o 300b,f,h,j,o 400b,c,k,s,p 500a,f,c,e,l,p,m,n Min support = 3 Frequent 1-items in frequency descending order: f,c,a,b,m,p {} f:4c:1 b:1 p:1 b:1c:3 a:3 b:1m:2 p:2m:1 Header Table Node Itemcounthead f1 c2 a1 b3 m2 p2

Frequent-pattern mining methods10 FP-Tree Construction Scans the database only twice Subsequent mining: based on the FP-tree

Frequent-pattern mining methods11 How to Mine an FP-tree? Step 1: form conditional pattern base Step 2: construct conditional FP-tree Step 3: recursively mine conditional FP- trees

Frequent-pattern mining methods12 Conditional Pattern Base Let {I} be a frequent item A sub database which consists of the set of prefix paths in the FP-tree With item {I} as a co-occurring suffix pattern Example: {m} is a frequent item {m}’s conditional pattern base: : support =2 : support = 1 Mine recursively on such databases {} f:4c:1 b:1 p:1 b:1c:3 a:3 b:1m:2 p:2m:1

Frequent-pattern mining methods13 Conditional Pattern Tree Let {I} be a suffix item, {DB|I} be the conditional pattern base The frequent pattern tree Tree I is known as the conditional pattern tree Example: {m} is a frequent item {m}’s conditional pattern base: : support =2 : support = 1 {m}’s conditional pattern tree {} f:4 c:3 a:3 m:2

Frequent-pattern mining methods14 Composition of patterns  and  Let  be a frequent item in DB, B be  ’s conditional pattern base, and  be an itemset in B. Then  +  is frequent in DB if and only if  is frequent in B. Example: Starting with  ={p} {p}’s conditional pattern base (from the tree) B= (f,c,a,m): 2 (c,b): 1 Let  be {c}. Then  ={p,c}, with support = 3.

Frequent-pattern mining methods15 Single path tree Let P be a single path FP tree Let {I 1, I 2, …I k } be an itemset in the tree Let I j have the lowest support Then the support({I 1, I 2, …I k })=support(I j ) Example: {} f:4c:1 b:1 p:1 b:1c:3 a:3 b:1m:2 p:2m:1

Frequent-pattern mining methods16 FP_growth Algorithm Fig 6.10 Recursive Algorithm Input: A transaction database, min_supp Output: The complete set of frequent patterns 1. FP-Tree construction 2. Mining FP-Tree by calling FP_growth(FP_tree, null) Key Idea: consider single path FP-tree and multi-path FP-tree separately Continue to split until get single-path FP-tree

Frequent-pattern mining methods17 FP_Growth (tree,  ) If tree contains a single path P, then For each combination (denoted as  ) of the nodes in the path P, then Generate pattern  +  with support = min_supp of nodes in  Else for each a in the header of tree, do { Generate pattern  = a +  with support = a. support ; Construct (1)  ’s conditional pattern base and (2)  ’s conditional FP-tree Tree  If Tree  is not empty, then Call FP-growth(Tree ,  ); }

Frequent-pattern mining methods18 FP-Growth vs. Apriori: Scalability With the Support Threshold Data set T25I20D10K

Frequent-pattern mining methods19 FP-Growth vs. Tree-Projection: Scalability with the Support Threshold Data set T25I20D100K

Frequent-pattern mining methods20 Why Is FP-Growth the Winner? Divide-and-conquer: decompose both the mining task and DB according to the frequent patterns obtained so far leads to focused search of smaller databases Other factors no candidate generation, no candidate test compressed database: FP-tree structure no repeated scan of entire database basic ops—counting and FP-tree building, not pattern search and matching

Frequent-pattern mining methods21 Implications of the Methodology: Papers by Han, et al. Mining closed frequent itemsets and max-patterns CLOSET (DMKD’00) Mining sequential patterns FreeSpan (KDD’00), PrefixSpan (ICDE’01) Constraint-based mining of frequent patterns Convertible constraints (KDD’00, ICDE’01) Computing iceberg data cubes with complex measures H-tree and H-cubing algorithm (SIGMOD’01)

Frequent-pattern mining methods22 Visualization of Association Rules: Pane Graph

Frequent-pattern mining methods23 Visualization of Association Rules: Rule Graph

Frequent-pattern mining methods24 Mining Various Kinds of Rules or Regularities Multi-level, quantitative association rules, correlation and causality, ratio rules, sequential patterns, emerging patterns, temporal associations, partial periodicity Classification, clustering, iceberg cubes, etc.

Frequent-pattern mining methods25 Multiple-level Association Rules Items often form hierarchy Flexible support settings: Items at the lower level are expected to have lower support. Transaction database can be encoded based on dimensions and levels explore shared multi-level mining uniform support Milk [support = 10%] 2% Milk [support = 6%] Skim Milk [support = 4%] Level 1 min_sup = 5% Level 2 min_sup = 5% Level 1 min_sup = 5% Level 2 min_sup = 3% reduced support

Frequent-pattern mining methods26 Quantitative Association Rules age(X,”34-35”)  income(X,”30K - 50K”)  buys(X,”high resolution TV”) Numeric attributes are dynamically discretized Such that the confidence or compactness of the rules mined is maximized. 2-D quantitative association rules: A quan1  A quan2  A cat Cluster “adjacent” association rules to form general rules using a 2-D grid. Example:

Frequent-pattern mining methods27 Redundant Rules [SA95] Which rule is redundant? milk  wheat bread, [support = 8%, confidence = 70%] “ skim milk ”  wheat bread, [support = 2%, confidence = 72%] The first rule is more general than the second rule. A rule is redundant if its support is close to the “ expected ” value, based on a general rule, and its confidence is close to that of the general rule.

INCREMENTAL MINING [CHNW96] Rules in DB were found and a set of new tuples db is added to DB, Task: to find new rules in DB + db. Usually, DB is much larger than db. Properties of Itemsets: frequent in DB + db if frequent in both DB and db. infrequent in DB + db if also in both DB and db. frequent only in DB, then merge with counts in db. No DB scan is needed! frequent only in db, then scan DB once to update their itemset counts. Same principle applicable to distributed/parallel mining.

Frequent-pattern mining methods29 CORRELATION RULES Association does not measure correlation [BMS97, AY98]. Among 5000 students 3000 play basketball, 3750 eat cereal, 2000 do both play basketball  eat cereal [40%, 66.7%] Conclusion: “ basketball and cereal are correlated ” is misleading because the overall percentage of students eating cereal is 75%, higher than 66.7%. Confidence does not always give correct picture!

Frequent-pattern mining methods30 Correlation Rules P(A^B)=P(B)*P(A), if A and B are independent events A and B negatively correlated  the value is less than 1; Otherwise A and B positively correlated. P(B|A)/P(B) is known as the lift of rule B  A If less than one, then B and A are negatively correlated. Basketball  Cereal 2000/(3000*3750/500 0)=2000*5000/3000*3 750<1

Frequent-pattern mining methods31 Chi-square Correlation [BMS97] The cutoff value at 95% significance level is 3.84 > 0.9 Thus, we do not reject the independence assumption.

Frequent-pattern mining methods32 Constraint-based Data Mining Finding all the patterns in a database autonomously? — unrealistic! The patterns could be too many but not focused! Data mining should be an interactive process User directs what to be mined using a data mining query language (or a graphical user interface) Constraint-based mining User flexibility: provides constraints on what to be mined System optimization: explores such constraints for efficient mining—constraint-based mining

Frequent-pattern mining methods33 Constraints in Data Mining Knowledge type constraint: classification, association, etc. Data constraint — using SQL-like queries find product pairs sold together in stores in Vancouver in Dec.’00 Dimension/level constraint in relevance to region, price, brand, customer category Rule (or pattern) constraint small sales (price $200) Interestingness constraint strong rules: min_support  3%, min_confidence  60%

Frequent-pattern mining methods34 Constrained Mining vs. Constraint-Based Search Constrained mining vs. constraint-based search/reasoning Both are aimed at reducing search space Finding all patterns satisfying constraints vs. finding some (or one) answer in constraint-based search in AI Constraint-pushing vs. heuristic search It is an interesting research problem on how to integrate them Constrained mining vs. query processing in DBMS Database query processing requires to find all Constrained pattern mining shares a similar philosophy as pushing selections deeply in query processing

Frequent-pattern mining methods35 Constrained Frequent Pattern Mining: A Mining Query Optimization Problem Given a frequent pattern mining query with a set of constraints C, the algorithm should be sound: it only finds frequent sets that satisfy the given constraints C complete: all frequent sets satisfying the given constraints C are found A naïve solution First find all frequent sets, and then test them for constraint satisfaction More efficient approaches: Analyze the properties of constraints comprehensively Push them as deeply as possible inside the frequent pattern computation.

Frequent-pattern mining methods36 Anti-Monotonicity in Constraint- Based Mining Anti-monotonicity intemset S satisfies the constraint, so does any of its subset sum(S.Price)  v is anti-monotone sum(S.Price)  v is not anti-monotone Example. C: range(S.profit)  15 is anti-monotone Itemset ab violates C So does every superset of ab TIDTransaction 10a, b, c, d, f 20b, c, d, f, g, h 30a, c, d, e, f 40c, e, f, g TDB (min_sup=2) ItemProfit a40 b0 c-20 d10 e-30 f30 g20 h-10

Frequent-pattern mining methods37 Which Constraints Are Anti- Monotone? ConstraintAntimonotone v  S No S  V no S  V yes min(S)  v no min(S)  v yes max(S)  v yes max(S)  v no count(S)  v yes count(S)  v no sum(S)  v ( a  S, a  0 ) yes sum(S)  v ( a  S, a  0 ) no range(S)  v yes range(S)  v no avg(S)  v,   { , ,  } convertible support(S)   yes support(S)   no

Frequent-pattern mining methods38 Monotonicity in Constraint- Based Mining Monotonicity When an intemset S satisfies the constraint, so does any of its superset sum(S.Price)  v is monotone min(S.Price)  v is monotone Example. C: range(S.profit)  15 Itemset ab satisfies C So does every superset of ab TIDTransaction 10a, b, c, d, f 20b, c, d, f, g, h 30a, c, d, e, f 40c, e, f, g TDB (min_sup=2) Ite m Profit a40 b0 c-20 d10 e-30 f30 g20 h-10

Frequent-pattern mining methods39 Which Constraints Are Monotone? ConstraintMonotone v  S yes S  V yes S  V no min(S)  v yes min(S)  v no max(S)  v no max(S)  v yes count(S)  v no count(S)  v yes sum(S)  v ( a  S, a  0 ) no sum(S)  v ( a  S, a  0 ) yes range(S)  v no range(S)  v yes avg(S)  v,   { , ,  } convertible support(S)   no support(S)   yes

Frequent-pattern mining methods40 Succinctness, Convertible, Inconvertable Constraints in Book We will not consider these in this course.

Frequent-pattern mining methods41 Associative Classification Mine association possible rules in form of itemset  class Itemset: a set of attribute-value pairs Class: class label Build Classifier Organize rules according to decreasing precedence based on confidence and support B. Liu, W. Hsu & Y. Ma. Integrating classification and association rule mining. In KDD’98

Frequent-pattern mining methods42 Classification by Aggregating Emerging Patterns Emerging pattern (EP): A pattern frequent in one class of data but infrequent in others. Age<=30 is frequent in class “buys_computer=yes” and infrequent in class “buys_computer=no” Rule: age<=30  buys computer G. Dong & J. Li. Efficient mining of emerging patterns: discovering trends and differences. In KDD’99