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Fuzzy Interpretation of Discretized Intervals Dr. Xindong Wu. Andrea Porter April 11, 2002. Plan For Presentation. Introduction to Problem, HCV Discretization Techniques/Fuzzy Borders A Hybrid Solution for HCV Experiments and Results Conclusion. Introduction.

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Presentation Transcript
plan for presentation
Plan For Presentation
  • Introduction to Problem, HCV
  • Discretization Techniques/Fuzzy Borders
  • A Hybrid Solution for HCV
  • Experiments and Results
  • Conclusion
  • Real-world data contains both numerical and nominal data, must be able to deal with different types of data.
  • Existing systems discretize numerical domains into intervals and treat intervals as nominal values during induction.
  • Problems occur if test examples are not covered in training data (no-match, multiple match)
  • The solution is a hybrid approach using fuzzy intervals for no-match problem.
  • Attribute based rule induction algorithm, extension matrix approach
    • Divide positive examples into intersecting groups
    • Find a heuristic conjunctive rule in each group that covers all PE and no NE
  • HCV can find a rule in the form of variable-valued logic
  • More compact than the decision trees/rules of ID3 and C4.5
variable valued logic and selectors
Variable Valued Logic and Selectors
  • Represents decisions where variables can take a range
  • Selector:

[ X # R ]

X = attribute

# = relational operator ( = , <, >, . . . )

R = Reference, list of 1 or more values

e.g [ Windy = true] [Temp > 90]

hcv software
HCV Software
  • C++ implementation
  • Can work with noisy and real-valued domains as well as nominal and noise-free databases
  • Provides a set of deduction facilities for the user to test the accuracy of the produced rules on test examples
c4 5 results vs hcv
C4.5:The T class

X2 = b

X1 = 0 & X3 = a

X1 = 0 & X3 = b

X1 = 0 & X2 = a

C4.5 Results vs. HCV
  • HCV:The T class
  • X2 = b
  • X1 = 0 & X2 = a
  • X1 = 0 & X4 = 0
  • C4.5:The F class

X1 = 1 & X2 = a

    • X1 = 1 & X2 = c
    • X2 = c & X3 = c
deduction of induction results
Deduction of Induction Results
  • Induction generates knowledge from existing data
  • Deduction applies induction results to interpret new data.
  • With real-world data, induction can not be assumed to be perfect
  • Three cases:

1) no-match (measure of fit)

2) single-match

3) multiple-match (estimate of probability)

  • Occurs during rule induction
  • Discretize numerical domains into intervals and treat similar to nominal values.
  • The challenge is to find the right borders for the intervals
  • Possible Methods:

1) Simplest Class-Separating Method

2) Information Gain Heuristic (implemented in HCV)

simplest class separating method
Simplest Class- Separating Method:
  • Interval Borders are places between each adjacent pair of examples which have different classes.
  • If attribute is very informative - method is efficient and useful.
  • If attribute is not informative - method produces too many intervals
information gain heuristic
Information Gain Heuristic

Use IGH to find more informative border.

  • x = (xi + xi+1)/2 for (i = 1, …, n-1)
  • x is a possible cut point if xi and xi+1 are of different classes.
  • Use IGH to find best x
  • Recursively split on left and right
  • To stop recursive splitting:

1) stop if IGH is same on all possible cut points.

2) stop if # of examples to split is less than a predefined number

3) limit the number of intervals

fuzzy borders
Fuzzy Borders
  • Discretization of continuous domains does not always fit accurate interpretation.
  • Instead of using sharp borders, use a membership function, measures the degree of membership.
  • A value can be classified into a few different intervals at the same time (e.g. single to multiple match)
fuzzy borders 2
Fuzzy Borders (2)
  • Fuzzy matching - deduction with fuzzy borders of discretized intervals.
  • Take the interval with the greatest degree as the value’s discrete value.
  • 3 functions to fuzzify borders:

1) linear

2) polynomial

3) arctan

  • Definitions

s = spread parameter l = length of original

xleft, xright = left/right sharp borders


xleft xright

linear membership function


xleft xright

Linear Membership Function

a = -kxleft + 1/2 b = kxright + 1/2

linleft(x) = kx + a

lin right(x) = -kx + b

lin(x) = MAX(0, MIN(1,linleft(x),linright(x)))

k = 1/2sl

polynomial membership function
*Polynomial Membership Function

polyside(x) = asidex3 + bsidex2 + csidex + dside

aside = 1/(4(ls)3)

bside = -3asidexside side {left,right}

cside = 3aside(xside2 - (ls)2)

dside = -a(xside3 -3xside(ls)2 + 2(ls)3)

polyleft(x), if xleft -ls  x  xleft + ls

poly(x) = polyright(x), if xright -ls  x  xright +ls

1, if xleft +ls  x  xright -ls

0, otherwise

match degree
Match Degree
  • Selector method - take the max membership degree of the value in all the intervals involved. If 2 adjacent intervals have the same class, values close to the border will have low membership.
  • Conjunction method - adds with fuzzy plus

ab=a + b - ab

no match resolution
No-Match Resolution

Largest Class

  • Assign all no match examples to the largest class, the default class.
  • Works well, if the number of classes in a training set is small and one class is clearly larger.
  • Deteriorates if there is a larger number of classes and the examples are evenly distributed
no match resolution1
No-Match Resolution

Measure of Fit

Calculate the measure of fit for each class:

1) calculate MF for each selector (sel)

MF(sel, e) = 1, if sel is satisfied by e

n/|x|, otherwise

2) calculate MF for each conjunctive rule(conj)

MF(conj, e) =  MF(sel, e) * n(conj)/N

no match resolution2
No-Match Resolution

Measure of Fit (2)

3) calculate MF for each class c

MF(c, e) = MF(conj1, e) + MF(conj2, e) - MF(conj1,e)MF(conj2,e)

* For more than two rules, apply formula recursively.

* Find maximum MF - determines which class is closest to the example

multiple match
  • Caused by over-generalization of the training examples at induction time
  • Example
    • (X1 = a, X2 = 1)
      • All PE cover X1 = a
      • All NE cover X2 = 1
      • Multiple Match
multiple match resolution
Multiple-Match Resolution

First Hit

  • Use first rule which classifies the example
  • Produces reasonable results if the rules from induction have been ordered according to a measure of reliability
  • Advantages - straightforward, efficient
  • Disadvantages - have to sort rules at induction time
multiple match resolution1
Multiple-Match Resolution

Largest Rule

  • Similar to largest class method from no-match resolution
  • Choose conjunctive rule that covers the most examples in the training set.
multiple match resolution2
Multiple-Match Resolution

Estimation of Probability

  • Assign EP value to each class based on the size of the satisfied conjunctive rules.

1) Find EP for each conjunctive rule (conj):

EP(conj, e)= { n(conj)/N, if conj is satisfied by e

0, otherwise

n(conj) = number of examples covered by conj

N = number of total examples

multiple match resolution3
Multiple-Match Resolution

Estimation of Probability (2)

2) Find EP value for each class:

EP(c, e) = EP(conj1, e) + EP(conj2, e) - EP(conj1,e)EP(conj2,e).

* For more rules, apply formula recursively

* Choose class with highest EP value

hybrid interpretation
Hybrid Interpretation
  • Used because fuzzy borders only add conflicts because they don’t reduce the number rules that are applicable
  • HCV - use sharp borders during induction and use fuzzy borders only during deduction
  • Algorithm:

* Single match - use class indicated by rules

* Multiple match - use estimation probability (EP) with sharp borders

* No match - use fuzzy borders with polynomial membership function to find closest rule

the data
The Data
  • Used 17 databases from the Machine Learning Database Repository, U. of California, Irvine.
  • Databases selected because:

1) All include numerical data

2) All lead to situations where no rules clearly apply.

results cont
Results (cont.)
  • The results shown for C4.5 and NewID are the pruned ones
    • These were usually better than the unpruned ones in this experiment
  • HCV did not fine tune different parameters because this would be loss of generality and applicability of the conclusions
accuracy results
Accuracy Results
  • HCV(hybrid) - 9 databases
  • C4.5 (R 8) - 7 databases
  • C4.5 (R 5) - 6 databases
  • HVC (large) - 3 databases
  • HCV (fuzzy) - 2 databases
hcv comparison
HCV Comparison
  • HCV (fuzzy) generally performs better than the simple largest class method
  • HCV’s performance improves significantly when the fuzzy borders (for no match) are combined with probability estimation (for multiple match) in HCV (hybrid)
  • Fuzzy borders are constructed and used at deduction time only when a no match case occurs.
  • This hybrid method performs more accurately than several other current deduction programs.
  • Fuzziness is strongly domain dependent, HCV allows the user to specify their own intervals and fuzzy functions.