03 preprocessing

1
Data Mining:
Concepts and Techniques
(3rd
ed.)
— Chapter 3 —
Jiawei Han, Micheline Kamber, and Jian Pei
University of Illinois at Urbana-Champaign &
Simon Fraser University
©2011 Han, Kamber & Pei. All rights reserved.

33
Chapter 3: Data Preprocessing
 Data Preprocessing: An Overview
 Data Quality
 Major Tasks in Data Preprocessing
 Data Cleaning
 Data Integration
 Data Reduction
 Data Transformation and Data Discretization
 Summary

4
Data Quality: Why Preprocess the Data?
 Measures for data quality: A multidimensional view
 Accuracy: correct or wrong, accurate or not
 Completeness: not recorded, unavailable, …
 Consistency: some modified but some not, dangling, …
 Timeliness: timely update?
 Believability: how trustable the data are correct?
 Interpretability: how easily the data can be
understood?

5
Major Tasks in Data Preprocessing
 Data cleaning
 Fill in missing values, smooth noisy data, identify or remove
outliers, and resolve inconsistencies
 Data integration
 Integration of multiple databases, data cubes, or files
 Data reduction
 Dimensionality reduction
 Numerosity reduction
 Data compression
 Data transformation and data discretization
 Normalization
 Concept hierarchy generation

66
 Data Quality
 Data Cleaning
 Data Reduction
 Summary

7
Data Cleaning
 Data in the Real World Is Dirty: Lots of potentially incorrect data, e.g.,
instrument faulty, human or computer error, transmission error
 incomplete: lacking attribute values, lacking certain attributes of
interest, or containing only aggregate data

e.g., Occupation=“ ” (missing data)
 noisy: containing noise, errors, or outliers

e.g., Salary=“−10” (an error)
 inconsistent: containing discrepancies in codes or names, e.g.,

Age=“42”, Birthday=“03/07/2010”

Was rating “1, 2, 3”, now rating “A, B, C”

discrepancy between duplicate records
 Intentional (e.g., disguised missing data)

Jan. 1 as everyone’s birthday?

8
Incomplete (Missing) Data
 Data is not always available
 E.g., many tuples have no recorded value for several
attributes, such as customer income in sales data
 Missing data may be due to
 equipment malfunction
 inconsistent with other recorded data and thus deleted
 data not entered due to misunderstanding
 certain data may not be considered important at the
time of entry
 not register history or changes of the data
 Missing data may need to be inferred

9
How to Handle Missing Data?
 Ignore the tuple: usually done when class label is missing
(when doing classification)—not effective when the % of
missing values per attribute varies considerably
 Fill in the missing value manually: tedious + infeasible?
 Fill in it automatically with
 a global constant : e.g., “unknown”, a new class?!
 the attribute mean
 the attribute mean for all samples belonging to the same
class: smarter
 the most probable value: inference-based such as
Bayesian formula or decision tree

10
Noisy Data
 Noise: random error or variance in a measured variable
 Incorrect attribute values may be due to
 faulty data collection instruments
 data entry problems
 data transmission problems
 technology limitation
 inconsistency in naming convention
 Other data problems which require data cleaning
 duplicate records
 incomplete data
 inconsistent data

11
How to Handle Noisy Data?
 Binning
 first sort data and partition into (equal-frequency) bins
 then one can smooth by bin means, smooth by bin
median, smooth by bin boundaries, etc.
 Regression
 smooth by fitting the data into regression functions
 Clustering
 detect and remove outliers
 Combined computer and human inspection
 detect suspicious values and check by human (e.g.,
deal with possible outliers)

12
Data Cleaning as a Process
 Data discrepancy detection
 Use metadata (e.g., domain, range, dependency, distribution)
 Check field overloading
 Check uniqueness rule, consecutive rule and null rule
 Use commercial tools

Data scrubbing: use simple domain knowledge (e.g., postal
code, spell-check) to detect errors and make corrections

Data auditing: by analyzing data to discover rules and
relationship to detect violators (e.g., correlation and clustering
to find outliers)
 Data migration and integration
 Data migration tools: allow transformations to be specified
 ETL (Extraction/Transformation/Loading) tools: allow users to
specify transformations through a graphical user interface
 Integration of the two processes
 Iterative and interactive (e.g., Potter’s Wheels)

1313
 Data Quality
 Data Cleaning
 Data Reduction
 Summary

1414
Data Integration
 Data integration:
 Combines data from multiple sources into a coherent store
 Schema integration: e.g., A.cust-id ≡ B.cust-#
 Integrate metadata from different sources
 Entity identification problem:
 Identify real world entities from multiple data sources, e.g., Bill
Clinton = William Clinton
 Detecting and resolving data value conflicts
 For the same real world entity, attribute values from different
sources are different
 Possible reasons: different representations, different scales, e.g.,
metric vs. British units

1515
Handling Redundancy in Data Integration
 Redundant data occur often when integration of multiple
databases
 Object identification: The same attribute or object may
have different names in different databases
 Derivable data: One attribute may be a “derived”
attribute in another table, e.g., annual revenue
 Redundant attributes may be able to be detected by
correlation analysis and covariance analysis
 Careful integration of the data from multiple sources may
help reduce/avoid redundancies and inconsistencies and
improve mining speed and quality

16
Correlation Analysis (Nominal Data)
 Χ2
(chi-square) test
 The larger the Χ2
value, the more likely the variables are
related
 The cells that contribute the most to the Χ2
value are those
whose actual count is very different from the expected
count
 Correlation does not imply causality
 # of hospitals and # of car-theft in a city are correlated
 Both are causally linked to the third variable: population
∑
−
=
Expected
ExpectedObserved 2
2 )(
χ

17
Chi-Square Calculation: An Example
 Χ2
(chi-square) calculation (numbers in parenthesis are
expected counts calculated based on the data distribution
in the two categories)
 It shows that like_science_fiction and play_chess are
correlated in the group
93.507
840
)8401000(
360
)360200(
210
)21050(
90
)90250( 2222
2
=
−
+
−
+
−
+
−
=χ
Play
chess
Not play chess Sum (row)
Like science fiction 250(90) 200(360) 450
Not like science fiction 50(210) 1000(840) 1050
Sum(col.) 300 1200 1500

18
Correlation Analysis (Numeric Data)
 Correlation coefficient (also called Pearson’s product
moment coefficient)
where n is the number of tuples, and are the respective means
of A and B, σA and σB are the respective standard deviation of A and
B, and Σ(aibi) is the sum of the AB cross-product.
 If rA,B > 0, A and B are positively correlated (A’s values
increase as B’s). The higher, the stronger correlation.
 rA,B = 0: independent; rAB < 0: negatively correlated
BA
n
i ii
BA
n
i ii
BA
n
BAnba
n
BbAa
r
σσσσ )1(
)(
)1(
))(( 11
,
−
−
=
−
−−
=
∑∑ ==
A B

19
Visually Evaluating Correlation
Scatter plots
showing the
similarity from
–1 to 1.

20
Correlation (viewed as linear relationship)
 Correlation measures the linear relationship
between objects
 To compute correlation, we standardize data
objects, A and B, and then take their dot product
)(/))((' AstdAmeanaa kk −=
)(/))((' BstdBmeanbb kk −=
''),( BABAncorrelatio •=

21
Covariance (Numeric Data)
 Covariance is similar to correlation
where n is the number of tuples, and are the respective mean or
expected values of A and B, σA and σB are the respective standard
deviation of A and B.
 Positive covariance: If CovA,B > 0, then A and B both tend to be larger
than their expected values.
 Negative covariance: If CovA,B < 0 then if A is larger than its expected
value, B is likely to be smaller than its expected value.
 Independence: CovA,B = 0 but the converse is not true:
 Some pairs of random variables may have a covariance of 0 but are not
independent. Only under some additional assumptions (e.g., the data follow
A B
Correlation coefficient:

Co-Variance: An Example
 It can be simplified in computation as
 Suppose two stocks A and B have the following values in one week:
(2, 5), (3, 8), (5, 10), (4, 11), (6, 14).
 Question: If the stocks are affected by the same industry trends, will
their prices rise or fall together?
 E(A) = (2 + 3 + 5 + 4 + 6)/ 5 = 20/5 = 4
 E(B) = (5 + 8 + 10 + 11 + 14) /5 = 48/5 = 9.6
 Cov(A,B) = (2×5+3×8+5×10+4×11+6×14)/5 − 4 × 9.6 = 4
 Thus, A and B rise together since Cov(A, B) > 0.

2323
 Data Quality
 Data Cleaning
 Data Reduction
 Summary

24
Data Reduction Strategies
 Data reduction: Obtain a reduced representation of the data set that
is much smaller in volume but yet produces the same (or almost the
same) analytical results
 Why data reduction? — A database/data warehouse may store
terabytes of data. Complex data analysis may take a very long time to
run on the complete data set.
 Data reduction strategies
 Dimensionality reduction, e.g., remove unimportant attributes

Wavelet transforms

Principal Components Analysis (PCA)

Feature subset selection, feature creation
 Numerosity reduction (some simply call it: Data Reduction)

Regression and Log-Linear Models

Histograms, clustering, sampling

Data cube aggregation

25
Data Reduction 1: Dimensionality Reduction
 Curse of dimensionality
 When dimensionality increases, data becomes increasingly sparse
 Density and distance between points, which is critical to clustering, outlier
analysis, becomes less meaningful
 The possible combinations of subspaces will grow exponentially
 Avoid the curse of dimensionality
 Help eliminate irrelevant features and reduce noise
 Reduce time and space required in data mining
 Allow easier visualization
 Dimensionality reduction techniques
 Wavelet transforms
 Principal Component Analysis
 Supervised and nonlinear techniques (e.g., feature selection)

26
Mapping Data to a New Space
Two Sine Waves Two Sine Waves + Noise Frequency
 Fourier transform
 Wavelet transform

27
What Is Wavelet Transform?
 Decomposes a signal into
different frequency subbands
 Applicable to n-
dimensional signals
 Data are transformed to
preserve relative distance
between objects at different
levels of resolution
 Allow natural clusters to
become more distinguishable
 Used for image compression

28
Wavelet Transformation
 Discrete wavelet transform (DWT) for linear signal
processing, multi-resolution analysis
 Compressed approximation: store only a small fraction of
the strongest of the wavelet coefficients
 Similar to discrete Fourier transform (DFT), but better
lossy compression, localized in space
 Method:
 Length, L, must be an integer power of 2 (padding with 0’s, when
necessary)
 Each transform has 2 functions: smoothing, difference
 Applies to pairs of data, resulting in two set of data of length L/2
 Applies two functions recursively, until reaches the desired length
Haar2 Daubechie4

29
Wavelet Decomposition
 Wavelets: A math tool for space-efficient hierarchical
decomposition of functions
 S = [2, 2, 0, 2, 3, 5, 4, 4] can be transformed to S^ = [23
/4,
-11
/4, 1
/2, 0, 0, -1, -1, 0]
 Compression: many small detail coefficients can be
replaced by 0’s, and only the significant coefficients are
retained

30
Why Wavelet Transform?
 Use hat-shape filters
 Emphasize region where points cluster
 Suppress weaker information in their boundaries
 Effective removal of outliers
 Insensitive to noise, insensitive to input order
 Multi-resolution
 Detect arbitrary shaped clusters at different scales
 Efficient
 Complexity O(N)
 Only applicable to low dimensional data

31
x2
x1
e
Principal Component Analysis (PCA)
 Find a projection that captures the largest amount of variation in data
 The original data are projected onto a much smaller space, resulting
in dimensionality reduction. We find the eigenvectors of the
covariance matrix, and these eigenvectors define the new space

32
 Given N data vectors from n-dimensions, find k ≤ n orthogonal vectors
(principal components) that can be best used to represent data
 Normalize input data: Each attribute falls within the same range
 Compute k orthonormal (unit) vectors, i.e., principal components
 Each input data (vector) is a linear combination of the k principal
component vectors
 The principal components are sorted in order of decreasing
“significance” or strength
 Since the components are sorted, the size of the data can be
reduced by eliminating the weak components, i.e., those with low
variance (i.e., using the strongest principal components, it is
possible to reconstruct a good approximation of the original data)
 Works for numeric data only
Principal Component Analysis (Steps)

33
Attribute Subset Selection
 Another way to reduce dimensionality of data
 Redundant attributes
 Duplicate much or all of the information contained in
one or more other attributes
 E.g., purchase price of a product and the amount of
sales tax paid
 Irrelevant attributes
 Contain no information that is useful for the data
mining task at hand
 E.g., students' ID is often irrelevant to the task of
predicting students' GPA

34
Heuristic Search in Attribute Selection
 There are 2d
possible attribute combinations of d attributes
 Typical heuristic attribute selection methods:
 Best single attribute under the attribute independence
assumption: choose by significance tests
 Best step-wise feature selection:

The best single-attribute is picked first

Then next best attribute condition to the first, ...
 Step-wise attribute elimination:

Repeatedly eliminate the worst attribute
 Best combined attribute selection and elimination
 Optimal branch and bound:

Use attribute elimination and backtracking

35
Attribute Creation (Feature Generation)
 Create new attributes (features) that can capture the
important information in a data set more effectively than
the original ones
 Three general methodologies
 Attribute extraction

Domain-specific
 Mapping data to new space (see: data reduction)

E.g., Fourier transformation, wavelet transformation,
manifold approaches (not covered)
 Attribute construction

Combining features (see: discriminative frequent
patterns in Chapter 7)

Data discretization

36
Data Reduction 2: Numerosity Reduction
 Reduce data volume by choosing alternative, smaller
forms of data representation
 Parametric methods (e.g., regression)
 Assume the data fits some model, estimate model
parameters, store only the parameters, and discard
the data (except possible outliers)
 Ex.: Log-linear models—obtain value at a point in m-D
space as the product on appropriate marginal
subspaces
 Non-parametric methods
 Do not assume models
 Major families: histograms, clustering, sampling, …

37
Parametric Data Reduction: Regression
and Log-Linear Models
 Linear regression
 Data modeled to fit a straight line
 Often uses the least-square method to fit the line
 Multiple regression
 Allows a response variable Y to be modeled as a
linear function of multidimensional feature vector
 Log-linear model
 Approximates discrete multidimensional probability
distributions

38
Regression Analysis
 Regression analysis: A collective name for
techniques for the modeling and analysis
of numerical data consisting of values of a
dependent variable (also called
response variable or measurement)
and of one or more independent variables
(aka. explanatory variables or
predictors)
 The parameters are estimated so as to
give a "best fit" of the data
 Most commonly the best fit is evaluated by
using the least squares method, but
other criteria have also been used
 Used for prediction
(including forecasting of
time-series data), inference,
hypothesis testing, and
modeling of causal
relationships
y
x
y = x + 1
X1
Y1
Y1’

39
 Linear regression: Y = w X + b
 Two regression coefficients, w and b, specify the line and are to be
estimated by using the data at hand
 Using the least squares criterion to the known values of Y1, Y2, …,
X1, X2, ….
 Multiple regression: Y = b0 + b1 X1 + b2 X2
 Many nonlinear functions can be transformed into the above
 Log-linear models:
 Approximate discrete multidimensional probability distributions
 Estimate the probability of each point (tuple) in a multi-dimensional
space for a set of discretized attributes, based on a smaller subset
of dimensional combinations
 Useful for dimensionality reduction and data smoothing
Regress Analysis and Log-Linear Models

40
Histogram Analysis
 Divide data into buckets and
store average (sum) for each
bucket
 Partitioning rules:
 Equal-width: equal bucket
range
 Equal-frequency (or equal-
depth)
0
5
10
15
20
25
30
35
40
10000
20000
30000
40000
50000
60000
70000
80000
90000
100000

41
Clustering
 Partition data set into clusters based on similarity, and
store cluster representation (e.g., centroid and diameter)
only
 Can be very effective if data is clustered but not if data is
“smeared”
 Can have hierarchical clustering and be stored in multi-
dimensional index tree structures
 There are many choices of clustering definitions and
clustering algorithms
 Cluster analysis will be studied in depth in Chapter 10

42
Sampling
 Sampling: obtaining a small sample s to represent the
whole data set N
 Allow a mining algorithm to run in complexity that is
potentially sub-linear to the size of the data
 Key principle: Choose a representative subset of the data
 Simple random sampling may have very poor
performance in the presence of skew
 Develop adaptive sampling methods, e.g., stratified
sampling:
 Note: Sampling may not reduce database I/Os (page at a
time)

43
Types of Sampling
 Simple random sampling
 There is an equal probability of selecting any particular
item
 Sampling without replacement
 Once an object is selected, it is removed from the
population
 Sampling with replacement
 A selected object is not removed from the population
 Stratified sampling:
 Partition the data set, and draw samples from each
partition (proportionally, i.e., approximately the same
percentage of the data)
 Used in conjunction with skewed data

44
Sampling: With or without Replacement
SRSWOR
(simple random
sample without
replacement)
SRSWR
Raw Data

45
Sampling: Cluster or Stratified Sampling
Raw Data Cluster/Stratified Sample

46
Data Cube Aggregation
 The lowest level of a data cube (base cuboid)
 The aggregated data for an individual entity of interest
 E.g., a customer in a phone calling data warehouse
 Multiple levels of aggregation in data cubes
 Further reduce the size of data to deal with
 Reference appropriate levels
 Use the smallest representation which is enough to
solve the task
 Queries regarding aggregated information should be
answered using data cube, when possible

47
Data Reduction 3: Data Compression
 String compression
 There are extensive theories and well-tuned algorithms
 Typically lossless, but only limited manipulation is
possible without expansion
 Audio/video compression
 Typically lossy compression, with progressive refinement
 Sometimes small fragments of signal can be
reconstructed without reconstructing the whole
 Time sequence is not audio
 Typically short and vary slowly with time
 Dimensionality and numerosity reduction may also be
considered as forms of data compression

48
Data Compression
Original Data Compressed
Data
lossless
Original Data
Approximated
lossy

49
 Data Quality
 Data Cleaning
 Data Reduction
 Summary

50
Data Transformation
 A function that maps the entire set of values of a given attribute to a
new set of replacement values s.t. each old value can be identified
with one of the new values
 Methods
 Smoothing: Remove noise from data
 Attribute/feature construction

New attributes constructed from the given ones
 Aggregation: Summarization, data cube construction
 Normalization: Scaled to fall within a smaller, specified range

min-max normalization

z-score normalization

normalization by decimal scaling
 Discretization: Concept hierarchy climbing

51
Normalization
 Min-max normalization: to [new_minA, new_maxA]
 Ex. Let income range $12,000 to $98,000 normalized to [0.0,
1.0]. Then $73,000 is mapped to
 Z-score normalization (μ: mean, σ: standard deviation):
 Ex. Let μ = 54,000, σ = 16,000. Then
 Normalization by decimal scaling
716.00)00.1(
000,12000,98
000,12600,73
=+−
−
−
AAA
AA
A
minnewminnewmaxnew
minmax
minv
v _)__(' +−
−
−
=
A
Av
v
σ
µ−
='
j
v
v
10
'= Where j is the smallest integer such that Max(|ν’|) < 1
225.1
000,16
000,54600,73
=
−

52
Discretization
 Three types of attributes
 Nominal—values from an unordered set, e.g., color, profession
 Ordinal—values from an ordered set, e.g., military or academic
rank
 Numeric—real numbers, e.g., integer or real numbers
 Discretization: Divide the range of a continuous attribute into intervals
 Interval labels can then be used to replace actual data values
 Reduce data size by discretization
 Supervised vs. unsupervised
 Split (top-down) vs. merge (bottom-up)
 Discretization can be performed recursively on an attribute
 Prepare for further analysis, e.g., classification

53
Data Discretization Methods
 Typical methods: All the methods can be applied
recursively
 Binning

Top-down split, unsupervised
 Histogram analysis

Top-down split, unsupervised
 Clustering analysis (unsupervised, top-down split or
bottom-up merge)
 Decision-tree analysis (supervised, top-down split)
 Correlation (e.g., χ2
) analysis (unsupervised, bottom-up
merge)

54
Simple Discretization: Binning
 Equal-width (distance) partitioning
 Divides the range into N intervals of equal size: uniform grid
 if A and B are the lowest and highest values of the attribute, the
width of intervals will be: W = (B –A)/N.
 The most straightforward, but outliers may dominate presentation
 Skewed data is not handled well
 Equal-depth (frequency) partitioning
 Divides the range into N intervals, each containing approximately
same number of samples
 Good data scaling
 Managing categorical attributes can be tricky

55
Binning Methods for Data Smoothing
 Sorted data for price (in dollars): 4, 8, 9, 15, 21, 21, 24, 25, 26, 28,
29, 34
* Partition into equal-frequency (equi-depth) bins:
- Bin 1: 4, 8, 9, 15
- Bin 2: 21, 21, 24, 25
- Bin 3: 26, 28, 29, 34
* Smoothing by bin means:
- Bin 1: 9, 9, 9, 9
- Bin 2: 23, 23, 23, 23
- Bin 3: 29, 29, 29, 29
* Smoothing by bin boundaries:
- Bin 1: 4, 4, 4, 15
- Bin 2: 21, 21, 25, 25
- Bin 3: 26, 26, 26, 34

56
Discretization Without Using Class Labels
(Binning vs. Clustering)
Data Equal interval width
(binning)
Equal frequency (binning) K-means clustering leads to better
results

57
Discretization by Classification &
Correlation Analysis
 Classification (e.g., decision tree analysis)
 Supervised: Given class labels, e.g., cancerous vs. benign
 Using entropy to determine split point (discretization point)
 Top-down, recursive split
 Details to be covered in Chapter 7
 Correlation analysis (e.g., Chi-merge: χ2
-based discretization)
 Supervised: use class information
 Bottom-up merge: find the best neighboring intervals (those
having similar distributions of classes, i.e., low χ2
values) to merge
 Merge performed recursively, until a predefined stopping condition

58
Concept Hierarchy Generation
 Concept hierarchy organizes concepts (i.e., attribute values)
hierarchically and is usually associated with each dimension in a data
warehouse
 Concept hierarchies facilitate drilling and rolling in data warehouses to
view data in multiple granularity
 Concept hierarchy formation: Recursively reduce the data by collecting
and replacing low level concepts (such as numeric values for age) by
higher level concepts (such as youth, adult, or senior)
 Concept hierarchies can be explicitly specified by domain experts
and/or data warehouse designers
 Concept hierarchy can be automatically formed for both numeric and
nominal data. For numeric data, use discretization methods shown.

59
Concept Hierarchy Generation
for Nominal Data
 Specification of a partial/total ordering of attributes
explicitly at the schema level by users or experts
 street < city < state < country
 Specification of a hierarchy for a set of values by explicit
data grouping
 {Urbana, Champaign, Chicago} < Illinois
 Specification of only a partial set of attributes
 E.g., only street < city, not others
 Automatic generation of hierarchies (or attribute levels) by
the analysis of the number of distinct values
 E.g., for a set of attributes: {street, city, state, country}

60
Automatic Concept Hierarchy Generation
 Some hierarchies can be automatically generated based on
the analysis of the number of distinct values per attribute in
the data set
 The attribute with the most distinct values is placed at
the lowest level of the hierarchy
 Exceptions, e.g., weekday, month, quarter, year
country
province_or_ state
city
street
15 distinct values
365 distinct values
3567 distinct values
674,339 distinct values

61
 Data Quality
 Data Cleaning
 Data Reduction
 Summary

62
Summary
 Data quality: accuracy, completeness, consistency, timeliness,
believability, interpretability
 Data cleaning: e.g. missing/noisy values, outliers
 Data integration from multiple sources:
 Entity identification problem
 Remove redundancies
 Detect inconsistencies
 Data reduction
 Numerosity reduction
 Data transformation and data discretization
 Normalization
 Concept hierarchy generation

63
References
 D. P. Ballou and G. K. Tayi. Enhancing data quality in data warehouse
environments. Comm. of ACM, 42:73-78, 1999
 T. Dasu and T. Johnson. Exploratory Data Mining and Data Cleaning. John
Wiley, 2003
 T. Dasu, T. Johnson, S. Muthukrishnan, V. Shkapenyuk.
Mining Database Structure; Or, How to Build a Data Quality Browser.
SIGMOD’02
 H. V. Jagadish et al., Special Issue on Data Reduction Techniques. Bulletin of
the Technical Committee on Data Engineering, 20(4), Dec. 1997
 D. Pyle. Data Preparation for Data Mining. Morgan Kaufmann, 1999
 E. Rahm and H. H. Do. Data Cleaning: Problems and Current Approaches.
IEEE Bulletin of the Technical Committee on Data Engineering. Vol.23, No.4
 V. Raman and J. Hellerstein. Potters Wheel: An Interactive Framework for Data
Cleaning and Transformation, VLDB’2001
 T. Redman. Data Quality: Management and Technology. Bantam Books, 1992
 R. Wang, V. Storey, and C. Firth. A framework for analysis of data quality
research. IEEE Trans. Knowledge and Data Engineering, 7:623-640, 1995

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