Transcript Introduction to Database Systems

Data Warehousing/Mining
Comp 150 DW
Chapter 3: Data Preprocessing
Instructor: Dan Hebert
Data Warehousing/Mining
1
Chapter 3: Data Preprocessing

Why preprocess the data?

Data cleaning

Data integration and transformation

Data reduction

Discretization and concept hierarchy generation

Summary
Data Warehousing/Mining
2
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
Data Warehousing/Mining
3
Multi-Dimensional Measure of Data
Quality

A well-accepted multidimensional view:
–
–
–
–
–
–
–
–

Accuracy
Completeness
Consistency
Timeliness
Believability
Value added
Interpretability
Accessibility
Broad categories:
– intrinsic, contextual, representational, and accessibility.
Data Warehousing/Mining
4
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
Data Warehousing/Mining
5
Forms of data preprocessing
Data Warehousing/Mining
6
Data Cleaning

Data cleaning tasks
– Fill in missing values
– Identify outliers and smooth out noisy data
– Correct inconsistent data
Data Warehousing/Mining
7
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.
Data Warehousing/Mining
8
Missing Data Example
Bank Acct Totals - Historical
Name
SSN
Address
Phone #
Date
Acct Total
John Doe
111-22-3333
1 Main St
Bedford,
Ma
111-222-3333
2/12/1999
2200.12
7/15/2000
12000.54
8/22/2001
2000.33
12/22/2002
15333.22
John W. Doe
Bedford,
Ma
John Doe
111-22-3333
James Smith
222-33-4444
2 Oak St
Boston, Ma
222-333-4444
Jim Smith
222-33-4444
2 Oak St
Boston, Ma
222-333-4444
Jim Smith
222-33-4444
2 Oak St
Boston, Ma
222-333-4444
How should we handle this?
Data Warehousing/Mining
12333.66
9
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: inferencebased such as Bayesian formula or decision tree
Data Warehousing/Mining
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 requires data cleaning
– duplicate records
– incomplete data
– inconsistent data
Data Warehousing/Mining
11
Noisy Data Example
Bank Acct Totals - Historical
Name
SSN
Address
Phone #
Date
Acct Total
John Doe
111-22-3333
1 Main St
Bedford,
Ma
111-222-3333
2/12/1999
2200.12
John Doe
111-22-3333
1 Main St
Bedford,
Ma
111-222-3333
2/12/1999
2233.67
James Smith
222-33-4444
2 Oak St
Boston, Ma
222-333-4444
12/22/2002
15333.22
James Smith
222-33-4444
2 Oak St
Boston, Ma
222-333-4444
12/23/2003
15333000.00
How should we handle this?
Data Warehousing/Mining
12
How to Handle Noisy Data?

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
– detect suspicious values and check by human

Regression
– smooth by fitting the data into regression functions
Data Warehousing/Mining
13
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
– Managing categorical attributes can be tricky.
Data Warehousing/Mining
14
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 (equi-width) bins:
- Bin 1 (4-14): 4, 8, 9
- Bin 2(15-24): 15, 21, 21, 24
- Bin 3(25-34): 25, 26, 28, 29, 34
* Smoothing by bin means:
- Bin 1: 7, 7, 7
- Bin 2: 20, 20, 20, 20
- Bin 3: 28, 28, 28, 28, 28
* Smoothing by bin boundaries:
- Bin 1: 4, 4, 4
- Bin 2: 15, 24, 24, 24
- Bin 3: 25, 25, 25, 25, 34
Data Warehousing/Mining
15
Binning Methods for Data
Smoothing (continued)
* 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:
- Bin 1: 4, 4, 4, 15
- Bin 2: 21, 21, 25, 25
- Bin 3: 26, 26, 26, 34
Data Warehousing/Mining
16
Cluster Analysis
Allows detection and removal of outliers
Data Warehousing/Mining
17
Regression
y
Y1
Y1’
y=x+1
X1
x
Linear regression – find the best line to fit two variables and
use regression function to smooth data
Data Warehousing/Mining
18
Data Integration

Data integration:
– combines data from multiple sources into a coherent store

Schema integration
– integrate metadata from different sources
– 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
– possible reasons: different representations, different scales, e.g.,
metric vs. British units
Data Warehousing/Mining
19
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
correlational analysis

Careful integration of the data from multiple sources
may help reduce/avoid redundancies and
inconsistencies and improve mining speed and quality
Data Warehousing/Mining
20
Correlational Analysis

R A,B = Sum (A-A’) (B-B’)
(n-1) sdAsdB
Where A’ = mean value of A
sum (A)
n
sdA = standard deviation of A
SqRoot ( Sum (A-A’)2)
n-1
<0 negatively correlated, =0 no correlation, >0 correlated –
consider removal of A or B
Data Warehousing/Mining
21
Correlational Analysis
Example






A – 2, 5, 6, 8, 22, 33, 44, 55
B – 6, 7, 22, 33, 44, 66, 67, 70
A’ = 22, B’ = 45
Sum (A-A’) = -1, Sum (B-B’) = -45
sdA = .378, sdB = 17.008
RA,B = 45/45.003 = .999
RA,B>0 - correlated – consider removal of A or B
Data Warehousing/Mining
22
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 (zero mean) normalization
– normalization by decimal scaling

Attribute/feature construction
– New attributes constructed from the given ones to help in the
data mining process
Data Warehousing/Mining
23
Data Transformation:
Normalization

min-max normalization
v  min A
v' 
(new _ max A  new _ min A)  new _ min A
max A  min A


Example – income, min $55,000, max $150000 – map to
0.0 – 1.0
$73,600 is transformed to :
– 73600-55000 (1.0 – 0) + 0 = 0.196
150000-55000
Data Warehousing/Mining
24
Data Transformation:
Normalization

z-score normalization
v  m eanA
v' 
stand _ devA


Example – income, mean $33000, sd $11000
$73600 is transformed to :
– 73600-33000 = 3.69
11000
Data Warehousing/Mining
25
Data Transformation:
Normalization

normalization by decimal scaling
v
v'  j
10


Where j is the smallest integer such that Max(| v ' |)<1
Example recorded values - -722 to 821
Divide each value by 1000
– -28 normalizes to -.028
– 444 normalizes to 0.444
Data Warehousing/Mining
26
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
Numerosity reduction
Discretization and concept hierarchy generation
Data Warehousing/Mining
27
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
Data Warehousing/Mining
28
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 (best & worst attributes determined
using various methods: statistical significance, info gain, …
Chpt 5 – more detail):
–
–
–
–
step-wise forward selection
step-wise backward elimination
combining forward selection and backward elimination
decision-tree induction
Data Warehousing/Mining
29
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
Data Warehousing/Mining
31
Data Compression
Compressed
Data
Original Data
lossless
Original Data
Approximated
Data Warehousing/Mining
32
Wavelet Transforms
Haar2
Daubechie4

Discrete wavelet transform (DWT): linear signal
processing

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 0s, 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
Data Warehousing/Mining
33
Principal Component Analysis

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
Data Warehousing/Mining
34
Numerosity Reduction

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
Data Warehousing/Mining
35
Regression and Log-Linear Models

Linear regression: Data are 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
Data Warehousing/Mining
36
Histograms
25
20
15
10
90000
80000
70000
60000
0
50000
5
100000
Data Warehousing/Mining
30
40000

35
30000

40
20000

A popular data
reduction technique
Divide data into buckets
and store average (sum)
for each bucket
Can be constructed
optimally in one
dimension using
dynamic programming
Related to quantization
problems.
10000

37
Histograms (continued)

Several techniques for determining buckets
– Equiwidth – width of each bucket range is uniform
– Equidepth – each bucket contains roughly the same
number of contiguous samples
– V-Optimal – weighted sum of the original values that
each bucket represents, where bucket weight =
number of values in a bucket
– MaxDiff – bucket boundary is established between
each pair for pairs having the B – 1 largest
differences, where B is user defined

V-Optimal & MaxDiff most accurate and
practical
Data Warehousing/Mining
38
Clustering

Partition data set into clusters, and one can store cluster
representation only

Can be very effective if data is clustered but not if data
is “smeared”

Can have hierarchical clustering and be stored in multidimensional index tree structures

There are many choices of clustering definitions and
clustering algorithms, further detailed in Chapter 8
Data Warehousing/Mining
39
Sampling


Allow a mining algorithm to run in complexity that is
potentially sub-linear to the size of the data
Choose a representative subset of the data
– Simple random sampling may have very poor performance in the
presence of skew

Develop adaptive sampling methods
– Stratified sampling:
 Approximate the percentage of each class (or subpopulation of
interest) in the overall database
 Used in conjunction with skewed data

Sampling may not reduce database I/Os (page at a time).
Data Warehousing/Mining
40
Sampling (continued)
All tuples have equal probability of selection
Once selected, can’t be
selected again
Raw Data
Data Warehousing/Mining
Once selected, can be
selected again
41
Sampling (continued)
If data is clustered or stratified, performed a simple random
sample (with or without replacement) in each cluster or strata
Raw Data
Data Warehousing/Mining
Cluster/Stratified Sample
42
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
Data Warehousing/Mining
43
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
– Some classification algorithms only accept categorical
attributes.
– Reduce data size by discretization
– Prepare for further analysis
Data Warehousing/Mining
44
Discretization and Concept hierarchy

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).
Data Warehousing/Mining
45
Discretization and concept hierarchy
generation for numeric data

Binning (see sections before)

Histogram analysis (see sections before)

Clustering analysis (see sections before)

Entropy-based discretization

Segmentation by natural partitioning
Data Warehousing/Mining
46
Entropy-Based Discretization

Given a set of samples S, if S is partitioned into
two intervals S1 and S2 using boundary T, the
entropy after partitioning is
E (S ,T ) 
| S1|
| S|
Ent ( S1) 
|S 2|
| S|
Ent ( S 2)
– S1 & S2 correspond to samples in S satisfying
conditions A<v & A>=v

The boundary that minimizes the entropy
function over all possible boundaries is selected
as a binary discretization.
Data Warehousing/Mining
47
Entropy-Based Discretization

The process is recursively applied to partitions
obtained until some stopping criterion is met,
e.g.,
Ent ( S )  E (T , S )  

Experiments show that it may reduce data size
and improve classification accuracy
Data Warehousing/Mining
48
Segmentation by natural
partitioning
3-4-5 rule can be used to segment numeric 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
* If it covers 2, 4, or 8 distinct values at the most
significant digit, partition the range into 4 intervals
* If it covers 1, 5, or 10 distinct values at the most
significant digit, partition the range into 5 intervals
Data Warehousing/Mining
49
Example of 3-4-5 rule
count
Step 1:
Step 2:
-$351
-$159
Min
Low (i.e, 5%-tile)
msd=1,000
profit
Low=-$1,000
(-$1,000 - 0)
(-$4000 - 0)
(-$2000 -$1000)
Data
Max
High=$2,000
($1,000 - $2,000)
(0 -$ 1,000)
(-$4000 -$5,000)
Step 4:
(-$3000 -$2000)
High(i.e, 95%-0 tile)
$4,700
(-$1,000 - $2,000)
Step 3:
(-$4000 -$3000)
$1,838
($1,000 - $2, 000)
(0 - $1,000)
(0 $200)
($1,000 $1,200)
($200 $400)
($1,200 $1,400)
($1,400 $1,600)
($400 $600)
(-$1000 0)
Warehousing/Mining
($600 $800)
($800 $1,000)
($1,600 ($1,800 $1,800)
$2,000)
($2,000 - $5, 000)
($2,000 $3,000)
($3,000 $4,000)
($4,000 $5,000)
50
Example of 3-4-5 rule (continued)





Step 1 – Min=-$351,976, Max=$4,700,896, low (5th percentile)=$159,876, high (95th percentile)=$1,838,761
Step 2 – For low and high, most significant digit is at $1,000,000,
rounding low -$1,000,000, rounding high $2,000,000
Step 3 – interval ranges over 3 distinct values at the most
significant digit, so using 3-4-5 rule partition into 3 intervals, $1,000,000-$0, $0-$1,000,000, and $1,000,000-$2,000,000
Step 4 – Examine Min & Max values to see how they “fit” into
first level partitions, first partition covers Min value, so adjust
left boundary to make partition smaller, last partition doesn’t
cover Max value, so create a new partition (round max up to
next significant digit) $2,000,000-$5,000,000
Step 5 – Recursively, each interval can be further partitioned
using 3-4-5 rule to form next lower level of the hierarchy
Data Warehousing/Mining
51
Concept hierarchy generation for
categorical data

Specification of a partial ordering of attributes
explicitly at the schema level by users or experts
– Example : rel db may contain: street, city, province_or_state,
country
– Expert defines ordering of hierarchy such as street < city <
province_or_state < country

Specification of a portion of a hierarchy by explicit
data grouping
– Example : province_or_state, country : {Alberta,
Saskatchewan, Manitoba} – prairies_Canada & {British
Columbia, prairies_Canada} – Western Canada
Data Warehousing/Mining
52
Concept hierarchy generation for
categorical data

Specification of a set of attributes, but not of their
partial ordering
– Auto generate the attribute ordering based upon observation
that attribute defining a high level concept has a smaller # of
distinct values than an attribute defining a lower level
concept
– Example : country (15), state_or_province (365), city (3567),
street (674,339)

Specification of only a partial set of attributes
– Try and parse database schema to determine
complete hierarchy
Data Warehousing/Mining
53
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
15 distinct values
province_or_ state
65 distinct values
city
3567 distinct values
Data Warehousing/Mining
street
674,339 distinct values
54
Summary

Data preparation is a big issue for both warehousing
and mining

Data preparation includes
– Data cleaning and data integration
– Data reduction and feature selection
– Discretization

A lot a methods have been developed but still an active
area of research
Data Warehousing/Mining
55
Homework Assignment

Design a data warehouse based on the hurricane data
provided (four years of data)
– Two data sources with slightly different formats and data



Utilize a star or snowflake schema
Implement the data warehouse utilizing relational
tables in postgres
Integrate the data from the two separate data sources
and import into your data warehouse
– Keep the data warehouse as you will utilize it in your next
homework assignment

Due March 18 at the start of class
Data Warehousing/Mining
56
Homework Assignment (cont)
Data Source 1
Table 1. Best track, Hurricane Alberto, 3 - 23 August 2000. Date/Time
(UTC) Position Pressure(mb) Wind Speed (kt) Stage Lat. (°N) Lon. (°W)
03/ 1800 10.8 18.0 1007 25 tropical depression
04 / 0000 11.5 20.1 1005 30 "
04 / 0600 12.0 22.3 1004 35 tropical storm
04 / 1200 12.3 23.8 1003 35 "
04 / 1800 12.7 25.2 1002 40 "
05 / 0000 13.2 26.7 1001 40 "
05 / 0600 13.7 28.2 1000 45 "
05 / 1200 14.1 29.8 999 45 "
05 / 1800 14.5 31.4 994 55 "
Data Warehousing/Mining
57
Homework Assignment (cont)
Data Source 2
Date: 04-23 AUG 2000
Hurricane ALBERTO
ADV LAT LON
TIME
WIND PR STAT
1 12.20 -22.70 08/04/09Z 30 1007 TROPICAL DEPRESSION
2 12.40 -25.00 08/04/15Z 35 1005 TROPICAL STORM
3 12.90 -25.90 08/04/21Z 50 999 TROPICAL STORM
4 13.40 -27.50 08/05/03Z 50 999 TROPICAL STORM
5 13.70 -28.70 08/05/09Z 55 994 TROPICAL STORM
6 14.40 -30.70 08/05/15Z 50 1000 TROPICAL STORM
7 14.70 -32.10 08/05/21Z 65 990 HURRICANE-1
Data Warehousing/Mining
58