Data preprocessing

Data Preprocessing
 Why preprocess the data?
 Data cleaning
 Data integration and transformation
 Data reduction
 Discretization and concept hierarchy
generation
 Summary

Why Data Preprocessing?
 Data in the real world is dirty
 incomplete: lacking attribute values, lacking certain
attributes of interest, or containing only aggregate data
 noisy: containing errors or outliers
 inconsistent: containing discrepancies in codes or
names
 No quality data, no quality mining results!
 Quality decisions must be based on quality data
 Data warehouse needs consistent integration of quality
data
 Required for both OLAP and Data Mining!

Why can Data be Incomplete?
 Attributes of interest are not available (e.g.,
customer information for sales transaction data)
 Data were not considered important at the time
of transactions, so they were not recorded!
 Data not recorder because of misunderstanding
or malfunctions
 Data may have been recorded and later deleted!
 Missing/unknown values for some data

Why can Data be Noisy/Inconsistent?
 Faulty instruments for data collection
 Human or computer errors
 Errors in data transmission
 Technology limitations (e.g., sensor data come at
a faster rate than they can be processed)
 Inconsistencies in naming conventions or data
codes (e.g., 2/5/2002 could be 2 May 2002 or 5
Feb 2002)
 Duplicate tuples, which were received twice
should also be removed

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 transformation
 Normalization and aggregation
 Data reduction
 Obtains reduced representation in volume but produces the
same or similar analytical results
 Data discretization
 Part of data reduction but with particular importance,
especially for numerical data
outliers=exceptions!

Data Cleaning
 Data cleaning tasks
 Fill in missing values
 Identify outliers and smooth out noisy data
 Correct inconsistent data

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

How to Handle Missing Data?
Age Income Religion Gender
23 24,200 Muslim M
39 ? Christian F
45 45,390 ? F
Fill missing values using aggregate functions (e.g., average) or
probabilistic estimates on global value distribution
E.g., put the average income here, or put the most probable income
based on the fact that the person is 39 years old
E.g., put the most frequent religion here

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

How to Handle Noisy Data?
Smoothing techniques
 Binning method:
 first sort data and partition into (equi-depth) bins
 then one can smooth by bin means, smooth by bin
median, smooth by bin boundaries, etc.
 Clustering
 detect and remove outliers
 Combined computer and human inspection
 computer detects suspicious values, which are then
checked by humans
 Regression
 smooth by fitting the data into regression functions
 Use Concept hierarchies
 use concept hierarchies, e.g., price value -> “expensive”

Simple Discretization Methods: Binning
 Equal-width (distance) partitioning:
 It 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:
 It divides the range into N intervals, each containing
approximately same number of samples
 Good data scaling – good handing of skewed data

Simple Discretization Methods: Binning
Example: customer ages
0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80
Equi-width
binning:
number
of values
0-22 22-31
44-4832-38
38-44 48-55
55-62
62-80
Equi-width
binning:

Smoothing using Binning Methods
* Sorted data for price (in dollars): 4, 8, 9, 15, 21, 21, 24,
25, 26, 28, 29, 34
* Partition into (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: [4,15],[21,25],[26,34]
- Bin 1: 4, 4, 4, 15
- Bin 2: 21, 21, 25, 25
- Bin 3: 26, 26, 26, 34

Cluster Analysis
cluster
outlier
salary
age

Regression
x
y
y = x + 1
X1
Y1
(salary)
(age)
Example of linear regression

Inconsistent Data
 Inconsistent data are handled by:
 Manual correction (expensive and tedious)
 Use routines designed to detect inconsistencies
and manually correct them. E.g., the routine may
use the check global constraints (age>10) or
functional dependencies
 Other inconsistencies (e.g., between names of
the same attribute) can be corrected during the
data integration process

Data Integration
 Data integration:
 combines data from multiple sources into a coherent store
 Schema integration
 integrate metadata from different sources
 metadata: data about the data (i.e., data descriptors)
 Entity identification problem: identify real world entities
from multiple data sources, e.g., A.cust-id ≡ B.cust-#
 Detecting and resolving data value conflicts
 for the same real world entity, attribute values from
different sources are different (e.g., J.D.Smith and Jonh
Smith may refer to the same person)
 possible reasons: different representations, different
scales, e.g., metric vs. British units (inches vs. cm)

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

Data Transformation
 Smoothing: remove noise from data
 Aggregation: summarization, data cube
construction
 Generalization: concept hierarchy climbing
 Normalization: scaled to fall within a small,
specified range
 min-max normalization
 z-score normalization
 normalization by decimal scaling
 Attribute/feature construction
 New attributes constructed from the given ones

Normalization: Why normalization?
 Speeds-up some learning techniques (ex.
neural networks)
 Helps prevent attributes with large ranges
outweigh ones with small ranges
 Example:
 income has range 3000-200000
 age has range 10-80
 gender has domain M/F

Data Transformation: Normalization
 min-max normalization
 e.g. convert age=30 to range 0-1, when
min=10,max=80. new_age=(30-10)/(80-10)=2/7
 z-score normalization
 normalization by decimal scaling
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minnewminnewmaxnew
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devstand_
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−
='
j
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'= Where j is the smallest integer such that Max(| |)<1'v

Data Reduction Strategies
 Warehouse may store terabytes of data: Complex
data analysis/mining may take a very long time to
run on the complete data set
 Data reduction
 Obtains a reduced representation of the data set that is
much smaller in volume but yet produces the same (or
almost the same) analytical results
 Data reduction strategies
 Data cube aggregation
 Dimensionality reduction
 Data compression
 Numerosity reduction
 Discretization and concept hierarchy generation

Data Cube Aggregation
 The lowest level of a data cube
 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

Dimensionality Reduction
 Feature selection (i.e., attribute subset selection):
 Select a minimum set of features such that the probability
distribution of different classes given the values for those
features is as close as possible to the original distribution
given the values of all features
 reduce # of patterns in the patterns, easier to understand
 Heuristic methods (due to exponential # of
choices):
 step-wise forward selection
 step-wise backward elimination
 combining forward selection and backward elimination
 decision-tree induction

Heuristic Feature Selection Methods
 There are 2d
possible sub-features of d features
 Several heuristic feature selection methods:
 Best single features under the feature independence
assumption: choose by significance tests.
 Best step-wise feature selection:
 The best single-feature is picked first
 Then next best feature condition to the first, ...
 Step-wise feature elimination:
 Repeatedly eliminate the worst feature
 Best combined feature selection and elimination:
 Optimal branch and bound:
 Use feature elimination and backtracking

Example of Decision Tree Induction
Initial attribute set:
{A1, A2, A3, A4, A5, A6}
A4 ?
A1? A6?
Class 1 Class 2 Class 1 Class 2
> Reduced attribute set: {A1, A4, A6}

Data Compression
Original Data Compressed
Data
lossless
Original Data
Approximated
lossy

 Given N data vectors from k-dimensions, find c
<= k orthogonal vectors that can be best used
to represent data
 The original data set is reduced to one consisting of N
data vectors on c principal components (reduced
dimensions)
 Each data vector is a linear combination of the c
principal component vectors
 Works for numeric data only
 Used when the number of dimensions is large
Principal Component Analysis or
Karhuren-Loeve (K-L) method

X1
X2
Y1
Y2
Principal Component Analysis
X1, X2: original axes (attributes)
Y1,Y2: principal components
significant component
(high variance)
Order principal components by significance and eliminate weaker ones

Numerosity Reduction:
Reduce the volume of data
 Parametric methods
 Assume the data fits some model, estimate model
parameters, store only the parameters, and discard the
data (except possible outliers)
 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

Histograms
 A popular data
reduction technique
 Divide data into
buckets and store
average (or sum) for
each bucket
 Can be constructed
optimally in one
dimension using
dynamic
programming
 Related to
quantization
problems.
0
5
10
15
20
25
30
35
40
10000 30000 50000 70000 90000

Histogram types
 Equal-width histograms:
 It divides the range into N intervals of equal size
 Equal-depth (frequency) partitioning:
 It divides the range into N intervals, each containing
approximately same number of samples
 V-optimal:
 It considers all histogram types for a given number of
buckets and chooses the one with the least variance.
 MaxDiff:
 After sorting the data to be approximated, it defines the
borders of the buckets at points where the adjacent
values have the maximum difference
 Example: split 1,1,4,5,5,7,9,14,16,18,27,30,30,32 to three
buckets
MaxDiff 27-18 and 14-9
Histograms

Clustering
 Partitions data set into clusters, and models it by
one representative from each cluster
 Can be very effective if data is clustered but not
if data is “smeared”
 There are many choices of clustering definitions
and clustering algorithms, further detailed in
Chapter 7

Cluster Analysis
cluster
outlier
salary
age
the distance between points in the
same cluster should be small
the distance between points in
different clusters should be large

Hierarchical Reduction
 Use multi-resolution structure with different
degrees of reduction
 Hierarchical clustering is often performed but tends
to define partitions of data sets rather than
“clusters”
 Parametric methods are usually not amenable to
hierarchical representation
 Hierarchical aggregation
 An index tree hierarchically divides a data set into
partitions by value range of some attributes
 Each partition can be considered as a bucket
 Thus an index tree with aggregates stored at each node is
a hierarchical histogram

Discretization
 Three types of attributes:
 Nominal — values from an unordered set
 Ordinal — values from an ordered set
 Continuous — real numbers
 Discretization:
 divide the range of a continuous attribute into
intervals
 why?
 Some classification algorithms only accept
categorical attributes.
 Reduce data size by discretization
 Prepare for further analysis

Discretization and Concept hierachy
 Discretization
 reduce the number of values for a given continuous
attribute by dividing the range of the attribute into
intervals. Interval labels can then be used to replace
actual data values.
 Concept hierarchies
 reduce the data by collecting and replacing low level
concepts (such as numeric values for the attribute age)
by higher level concepts (such as young, middle-aged,
or senior).

Discretization and concept hierarchy
generation for numeric data
 Binning/Smoothing (see sections before)
 Histogram analysis (see sections before)
 Clustering analysis (see sections before)
 Entropy-based discretization
 Segmentation by natural partitioning

Entropy-Based Discretization
 Given a set of samples S, if S is partitioned into
two intervals S1 and S2 using boundary T, the
information gain I(S,T) after partitioning is
 The boundary that maximizes the information gain
over all possible boundaries is selected as a binary
discretization.
 The process is recursively applied to partitions
obtained until some stopping criterion is met, e.g.,
 Experiments show that it may reduce data size
and improve classification accuracy
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Segmentation by natural partitioning
The 3-4-5 rule can be used to segment numerical data into
relatively uniform, “natural” intervals.
* If an interval covers 3, 6, 7 or 9 distinct values at the most
significant digit, partition the range into 3 equiwidth intervals
for 3,6,9 or 2-3-2 for 7
* If it covers 2, 4, or 8 distinct values at the most significant
digit, partition the range into 4 equiwidth intervals
* If it covers 1, 5, or 10 distinct values at the most significant
digit, partition the range into 5 equiwidth intervals
 Users often like to see numerical ranges partitioned into
relatively uniform, easy-to-read intervals that appear intuitive
or “natural”. E.g., [50-60] better than [51.223-60.812]
The rule can be recursively applied for the resulting intervals

Concept hierarchy generation for
categorical data
 Categorical attributes: finite, possibly large domain, with no
ordering among the values
 Example: item type
 Specification of a partial ordering of attributes explicitly at
the schema level by users or experts
 Example: location is split by domain experts to
street<city<state<country
 Specification of a portion of a hierarchy by explicit data
grouping
 Specification of a set of attributes, but not of their partial
ordering
 Specification of only a partial set of attributes

Specification of a set of attributes
Concept hierarchy can be automatically
generated based on the number of distinct
values per attribute in the given attribute set.
The attribute with the most distinct values is
placed at the lowest level of the hierarchy.
country
province_or_ state
city
street
15 distinct values
65 distinct
values
3567 distinct values
674,339 distinct values

Data preprocessing

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Data preprocessing