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Mining Association Rules

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1 Mining Association Rules
Part II

2 Data Mining Overview Data Mining
Data warehouses and OLAP (On Line Analytical Processing.) Association Rules Mining Clustering: Hierarchical and Partitional approaches Classification: Decision Trees and Bayesian classifiers Sequential Patterns Mining Advanced topics: outlier detection, web mining

3 Problem Statement I = {i1, i2, …, im}: a set of literals, called items
Transaction T: a set of items s.t. T I Database D: a set of transactions A transaction contains X, a set of items in I, if X T An association rule is an implication of the form X  Y, where X,Y I The rule X  Y holds in the transaction set D with confidence c if c% of transactions in D that contain X also contain Y The rule X  Y has support s in the transaction set D if s% of transactions in D contain X Y Find all rules that have support and confidence greater than user-specified min support and min confidence

4 Problem Decomposition
1. Find all sets of items that have minimum support (frequent itemsets) 2. Use the frequent itemsets to generate the desired rules

5 Mining Frequent Itemsets: the Key Step
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.

6 The Apriori Algorithm Lk: Set of frequent itemsets of size k (those with min support) Ck: Set of candidate itemset of size k (potentially frequent itemsets) 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 that are contained in t Lk+1 = candidates in Ck+1 with min_support end return k Lk;

7 How to Generate Candidates?
Suppose the items in Lk-1 are listed in 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.item1=q.item1, …, p.itemk-2=q.itemk-2, p.itemk-1 < 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

8 How to Count Supports of Candidates?
Why counting supports of candidates a problem? The total number of candidates can be very huge One transaction may contain many candidates Method: Candidate itemsets are stored in a hash-tree Leaf node of hash-tree contains a list of itemsets and counts Interior node contains a hash table Subset function: finds all the candidates contained in a transaction

9 Hash-tree:search Given a transaction T and a set Ck find all of its members contained in T Assume an ordering on the items Start from the root, use every item in T to go to the next node If you are at an interior node and you just used item i, then use each item that comes after i in T If you are at a leaf node check the itemsets

10 Is Apriori Fast Enough? — Performance Bottlenecks
The core of the Apriori algorithm: Use frequent (k – 1)-itemsets to generate candidate frequent k-itemsets Use database scan and pattern matching to collect counts for the candidate itemsets The bottleneck of Apriori: candidate generation Huge candidate sets: 104 frequent 1-itemset will generate 107 candidate 2-itemsets To discover a frequent pattern of size 100, e.g., {a1, a2, …, a100}, one needs to generate 2100  1030 candidates. Multiple scans of database: Needs (n +1 ) scans, n is the length of the longest pattern

11 Max-Miner Max-miner finds long patterns efficiently: the maximal frequent patterns Instead of checking all subsets of a long pattern try to detect long patterns early Scales linearly to the size of the patterns

12 Max-Miner: the idea f Pruning: (1) set infrequency
Set enumeration tree of an ordered set f 1 2 3 4 Pruning: (1) set infrequency (2) Superset frequency 1,2 1,3 1,4 2,3 2,4 3,4 1,2,3 1,2,4 1,3,4 2,3,4 Each node is a candidate group g h(g) is the head: the itemset of the node t(g) tail: an ordered set that contains all items that can appear in the subnodes 1,2,3,4 Example: h({1}) = {1} and t({1}) = {2,3,4}

13 Max-miner pruning When we count the support of a candidate group g, we compute also the support for h(g), h(g) t(g) and h(g) {i} for each i in t(g) If h(g) t(g) is frequent, then stop expanding the node g and report the union as frequent itemset If h(g) {i} is infrequent, then remove i from all subnodes (just remove i from any tail of a group after g) Expand the node g by one and do the same

14 The algorithm Max-Miner Set candidate groups C {}
Set of Itemsets F {Gen-Initial-Groups(T,C)} while C not empty do scan T to count the support of all candidate groups in C for each g in C s.t. h(g) U t(g) is frequent do F  F U {h(g) U t(g)} Set candidate groups Cnew{ } for each g in C such that h(g) U t(g) is infrequent do F F U {Gen-sub-nodes(g, Cnew)} C  Cnew remove from F any itemset with a proper superset in F remove from C any group g s.t. h(g) U t(g) has a superset in F return F

15 The algorithm (2) Gen-Initial-Groups(T, C)
scan T to obtain F1, the set of frequent 1-itemsets impose an ordering on items in F1 for each item i in F1 other than the greatest itemset do let g be a new candidate with h(g) = {i} and t(g) = {j | j follows i in the ordering} C C U {g} return the itemset F1 (an the C of course) Gen-sub-nodes(g, C) /* generation of new itemsets at the next level*/ remove any item i from t(g) if h(g) U {i} is infrequent reorder the items in t(g) for each i in t(g) other than the greatest do let g’ be a new candidate with h(g’) = h(g) U {i} and t(g’) = {j | j in t(g) and j is after i in t(g)} C  C U {g’} return h(g) U {m} where m is the greatest item in t(g) or h(g) if t(g) is empty

16 Item Ordering By re-ordering items we try to increase the effectiveness of frequency-pruning Very frequent items have higher probability to be contained in long patterns Put these item at the end of the ordering, so they appear in many tails

17 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!

18 Construct FP-tree from a Transaction DB
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} {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} min_support = 0.5 {} f:4 c:1 b:1 p:1 c:3 a:3 m:2 p:2 m:1 Header Table Item frequency head f 4 c 4 a 3 b 3 m 3 p 3 Steps: Scan DB once, find frequent 1-itemset (single item pattern) Order frequent items in frequency descending order Scan DB again, construct FP-tree

19 Benefits of the FP-tree Structure
Completeness: never breaks a long pattern of any transaction preserves complete information for frequent pattern mining Compactness reduce irrelevant information—infrequent items are gone frequency descending ordering: more frequent items are more likely to be shared never be larger than the original database (if not count node-links and counts) Example: For Connect-4 DB, compression ratio could be over 100

20 Mining Frequent Patterns Using FP-tree
General idea (divide-and-conquer) Recursively grow frequent pattern path using the FP-tree Method For each 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)

21 Major Steps to Mine FP-tree
Construct conditional pattern base for each node in the FP-tree Construct conditional FP-tree from each conditional pattern-base Recursively mine conditional FP-trees and grow frequent patterns obtained so far If the conditional FP-tree contains a single path, simply enumerate all the patterns

22 Step 1: From FP-tree to Conditional Pattern Base
Starting at the frequent header table in the FP-tree Traverse the FP-tree by following the link of each frequent item Accumulate all of transformed prefix paths of that item to form a conditional pattern base {} f:4 c:1 b:1 p:1 c:3 a:3 m:2 p:2 m:1 Header Table Item frequency head f 4 c 4 a 3 b 3 m 3 p 3 Conditional pattern bases item cond. 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

23 Properties of FP-tree for Conditional Pattern Base Construction
Node-link property For any frequent item ai, all the possible frequent patterns that contain ai can be obtained by following ai's node-links, starting from ai's head in the FP-tree header Prefix path property To calculate the frequent patterns for a node ai in a path P, only the prefix sub-path of ai in P need to be accumulated, and its frequency count should carry the same count as node ai.

24 Step 2: Construct Conditional FP-tree
For each pattern-base Accumulate the count for each item in the base Construct the FP-tree for the frequent items of the pattern base {} m-conditional pattern base: fca:2, fcab:1 Header Table Item frequency head f 4 c 4 a 3 b 3 m 3 p 3 f:4 c:1 All frequent patterns concerning m m, fm, cm, am, fcm, fam, cam, fcam {} f:3 c:3 a:3 m-conditional FP-tree c:3 b:1 b:1 a:3 p:1 m:2 b:1 p:2 m:1

25 Mining Frequent Patterns by Creating Conditional Pattern-Bases
Empty f {(f:3)}|c {(f:3)} c {(f:3, c:3)}|a {(fc:3)} a {(fca:1), (f:1), (c:1)} b {(f:3, c:3, a:3)}|m {(fca:2), (fcab:1)} m {(c:3)}|p {(fcam:2), (cb:1)} p Conditional FP-tree Conditional pattern-base Item

26 Step 3: Recursively mine the conditional FP-tree
{} f:3 c:3 am-conditional FP-tree {} f:3 c:3 a:3 m-conditional FP-tree Cond. pattern base of “am”: (fc:3) {} Cond. pattern base of “cm”: (f:3) f:3 cm-conditional FP-tree {} Cond. pattern base of “cam”: (f:3) f:3 cam-conditional FP-tree

27 Single FP-tree Path Generation
Suppose an FP-tree T has a single path P The complete set of frequent pattern of T can be generated by enumeration of all the combinations of the sub-paths of P {} All frequent patterns concerning m m, fm, cm, am, fcm, fam, cam, fcam f:3 c:3 a:3 m-conditional FP-tree

28 Principles of Frequent Pattern Growth
Pattern growth property Let  be a frequent itemset in DB, B be 's conditional pattern base, and  be an itemset in B. Then    is a frequent itemset in DB iff  is frequent in B. “abcdef ” is a frequent pattern, if and only if “abcde ” is a frequent pattern, and “f ” is frequent in the set of transactions containing “abcde ”

29 Why Is Frequent Pattern Growth Fast?
Our 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

30 FP-growth vs. Apriori: Scalability With the Support Threshold
Data set T25I20D10K

31 FP-growth vs. Tree-Projection: Scalability with Support Threshold
Data set T25I20D100K

32 Presentation of Association Rules (Table Form )

33 Visualization of Association Rule Using Plane Graph

34 Visualization of Association Rule Using Rule Graph

35 Iceberg Queries Icerberg query: Compute aggregates over one or a set of attributes only for those whose aggregate values is above certain threshold Example: select P.custID, P.itemID, sum(P.qty) from purchase P group by P.custID, P.itemID having sum(P.qty) >= 10 Compute iceberg queries efficiently by Apriori: First compute lower dimensions Then compute higher dimensions only when all the lower ones are above the threshold

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