SEQUENTIAL

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SEQUENTIAL PATTERNS THE GSP ALGORITHM

• BY JOE CASABONA

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INTRO

• What are Sequential Patterns?
• Why don't ARs suffice?
• The General Sequential Pattern Algorithm
• Finding Frequent Sets
• Candidate Generation
• Rule Generation

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WHAT ARE SEQUENTIAL PATTERNS?

• "Finding statistically relevant patterns between
  data examples where the values are delivered in a
  sequence." 3
• Very similar to Association Rules, but sequence
  in this case matters.
•  
• There may be times when order is important. 

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SEQUENTIAL PATTERN EXAMPLES

• In Transaction Processing 
•     Do customers usually buy a new controller or
  a game first after buying an Xbox?
• In Text Mining
•     Order of the words important for finding
  linguistic or
• language patterns 1

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OBJECTIVE

• Given a set S of input data sequences, find all
  sequences that have a user-specified minimum
  support. This is called a 'frequent sequence' or
  sequential pattern. 1
• We will use the Generalized Sequential Pattern
  Algorithm (GSP)

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GSP

• Similar to Apriori Algorithm
•  Find individual items with minSupport
  (1-sequences)
• Use them to find 2-sequences
• Continue using k-sequences to find
  (k1)-sequences
• Stop when there are no more frequent sequences.
•  
• Difference is in Candidate Generation 

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GSP CANDIDATE GENERATION

• Input Frequent Set k-1 (Fk-1)
• Output Candidate Set Ck
•  
• How it works
•  
• Join Fk-1 with Fk-1
•  Get rid of infrequent sequences (prune)
• Note Order of items matter 

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CANDIDATE EXAMPLE

• F3 lt1, 2 4gt, lt1, 2 5gt, lt1 4, 5gt,
  lt1, 4 6gt, lt2 4, 5gt, lt2 4 6gt
•  
• After Join lt1, 2 4, 5gt, lt1, 2 4 6gt
•  
• After Prune lt1, 2 4, 5gt 
•  
• C4  lt1, 2 4, 5gt

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RULE GENERATION

• Objective not to generate rules, but it can be
  done. 
•  
• Sequential Rule Apply confidence to  Frequent
  Sequences
•  
• Label Sequential Rules Replace some elements in
  X with

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RERERENCES

• 1 The Book I am using
•  Liu, Bing. Web Data Mining, Chapter 2
  Association Rules and Sequential Patterns.
  Springer, December, 2006 
•  
• Wikipedia
• 2 "GSP Algorithm." http//en.wikipedia.org/wiki/
  GSP_AlgorithmJune 3, 2008
• 3 "Sequence Mining." http//en.wikipedia.org/wi
  ki/Sequence_miningOct. 30, 2008