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7. Sequence Mining. Sequences and Strings Recognition with Strings MM & HMM Sequence Association Rules. Sequences and Strings. A sequence x is an ordered list of discrete items, such as a sequence of letters or a gene sequence Sequences and strings are often used interchangeably

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7 sequence mining

7. Sequence Mining

Sequences and Strings

Recognition with Strings

MM & HMM

Sequence Association Rules

Data Mining by H. Liu, ASU


Sequences and strings
Sequences and Strings

  • A sequence x is an ordered list of discrete items, such as a sequence of letters or a gene sequence

    • Sequences and strings are often used interchangeably

    • String elements (characters, letters, or symbols) are nominal

    • A particularly long string is called text

  • |x| denotes the length of sequence x

    • |AGCTTC| is 6

  • Any contiguous string that is part of x is called a substring, segment, or factor of x

    • GCT is a factor of AGCTTC

Data Mining by H. Liu, ASU


Recognition with strings
Recognition with Strings

  • String matching

    • Given x and text, determine whether x is a factor of text

  • Edit distance

    • Given two strings x and y, compute the min number of basic operations (character insertions, deletions and exchanges) needed to transform x into y

Data Mining by H. Liu, ASU


String matching
String Matching

  • Given |text| >> |x|, each discrete character is taken from an alphabet A

    • A can be {0, 1}, {0, 1, 2,…, 9}, {A,G,C,T}, or {A, B,…}

  • A shift s is an offset needed to align the first character of x with character number s+1 in text

  • Find if there exists a valid shift where there is a perfect match between each character in x and the corresponding one in text

Data Mining by H. Liu, ASU


Na ve string matching
Naïve String Matching

  • Given alphabet A, x, text, n = |text|, m = |x|

    s = 0

    whiles ≤ n-m

    ifx[1 …m] = text [s+1 … s+m]

    then print “pattern occurs at shift” s

    s = s + 1

  • Time complexity (worst case): O((n-m+1)m)

  • One character shift at a time is not necessary

Data Mining by H. Liu, ASU


Boyer moore string matching
Boyer-Moore String Matching

  • Given A, x, text, n = |text|, m = |x|

    F(x) = last-occurrence function

    G(x) = good-suffix function; s = 0

    whiles ≤ n-m

    j = m

    while j>0 andx[j] = text [s+j]

    j = j-1

    if j = 0

    then print “pattern occurs at shift” s

    s = s + G(0)

    else s = s + max[G(j), j-F(text[s+j0])]

Data Mining by H. Liu, ASU


Edit distance
Edit Distance

  • ED between x and y describes how many fundamental operations are required to transform x to y.

  • Fundamental operations (x=‘excused’, y=‘exhausted’)

    • Substitutions, ‘c’ is replaced by ‘h’

    • Insertions, ‘a’ is inserted into x after ‘h’

    • Deletions, a character in x is deleted

  • ED is one way of measuring similarity between two strings

Data Mining by H. Liu, ASU


Classification using ed
Classification using ED

  • Nearest-neighbor algorithm can be applied for pattern recognition.

    • Training: data of strings with their class labels stored

    • Classification (testing): a test string is compared to each stored string and an ED is computed; the nearest stored string’s label is assigned to the test string.

  • The key is how to calculate ED.

  • An example of calculating ED

Data Mining by H. Liu, ASU


Hidden markov model
Hidden Markov Model

  • Markov Model: transitional states

  • Hidden Markov Model: additional visible states

  • Evaluation

  • Decoding

  • Learning

Data Mining by H. Liu, ASU


Markov model
Markov Model

  • The Markov property:

    • given the current state, the transition probability is independent of any previous states.

  • A simple Markov Model

    • State ω(t) at time t

    • Sequence of length T:

      • ωT = {ω(1), ω(2), …, ω(T)}

    • Transition probability

      • P(ωj(t+1)| ωi(t)) = aij

    • It’s not required that aij =aji

Data Mining by H. Liu, ASU


Hidden markov model1
Hidden Markov Model

  • Visible states

    • VT = {v(1), v(2), …, v(T)}

  • Emitting a visible state vk(t)

    • P(v k(t)| ωj(t)) = bjk

  • Only visible states vk (t) are accessibleand states ωi (t) are unobservable.

  • A Markov model is ergodic if every state has a nonzero prob of occuring give some starting state.

Data Mining by H. Liu, ASU


Three key issues with hmm
Three Key Issues with HMM

  • Evaluation

    • Given an HMM, complete with transition probabilities aij and bjk. Determine the probability that a particular sequence of visible states VT was generated by that model

  • Decoding

    • Given an HMM and a set of observations VT. Determine the most likely sequence of hidden states ωT that led to VT.

  • Learning

    • Given the number of states and visible states and a set of training observations of visible symbols, determine the probabilities aij and bjk.

Data Mining by H. Liu, ASU


Sequence association rule mining
Sequence Association Rule Mining

  • SPADE (Sequential Pattern Discovery using Equivalence classes)

  • Constrained sequence mining (SPIRIT)

Data Mining by H. Liu, ASU


Bibliography
Bibliography

  • R.O. Duda, P.E. Hart, and D.G. Stork, 2001. Pattern Classification. 2nd Edition. Wiley Interscience.

Data Mining by H. Liu, ASU


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