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the purchaser’s name and address instead of a key to this information in a purchaser
database, discrepancies can occur, such as the same purchaser’s name appearing with
different addresses within the purchase order database.
3.3.4
Data Value Conflict Detection and Resolution
Data integration also involves the detection and resolution of data value conflicts. For
example, for the same real-world entity, attribute values from different sources may dif-
fer. This may be due to differences in representation, scaling, or encoding. For instance,
a weight attribute may be stored in metric units in one system and British imperial
units in another. For a hotel chain, the price of rooms in different cities may involve
not only different currencies but also different services (e.g., free breakfast) and taxes.
When exchanging information between schools, for example, each school may have its
own curriculum and grading scheme. One university may adopt a quarter system, offer
three courses on database systems, and assign grades from A+ to F, whereas another
may adopt a semester system, offer two courses on databases, and assign grades from 1
to 10. It is difficult to work out precise course-to-grade transformation rules between
the two universities, making information exchange difficult.
Attributes may also differ on the abstraction level, where an attribute in one sys-
tem is recorded at, say, a lower abstraction level than the “same” attribute in another.
For example, the total sales in one database may refer to one branch of All Electronics,
while an attribute of the same name in another database may refer to the total sales
for All Electronics stores in a given region. The topic of discrepancy detection is further
described in Section 3.2.3 on data cleaning as a process.
3.4
Data Reduction
Imagine that you have selected data from the AllElectronics data warehouse for analysis.
The data set will likely be huge! Complex data analysis and mining on huge amounts of
data can take a long time, making such analysis impractical or infeasible.
Data reduction techniques can be applied to obtain a reduced representation of the
data set that is much smaller in volume, yet closely maintains the integrity of the original
data. That is, mining on the reduced data set should be more efficient yet produce the
same (or almost the same) analytical results. In this section, we first present an overview
of data reduction strategies, followed by a closer look at individual techniques.
3.4.1
Overview of Data Reduction Strategies
Data reduction strategies include dimensionality reduction, numerosity reduction, and
data compression.
Dimensionality reduction is the process of reducing the number of random variables
or attributes under consideration. Dimensionality reduction methods include wavelet
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transforms (Section 3.4.2) and
principal components analysis (Section 3.4.3), which
transform or project the original data onto a smaller space. Attribute subset selection is a
method of dimensionality reduction in which irrelevant, weakly relevant, or redundant
attributes or dimensions are detected and removed (Section 3.4.4).
Numerosity reduction techniques replace the original data volume by alternative,
smaller forms of data representation. These techniques may be parametric or non-
parametric. For parametric methods, a model is used to estimate the data, so that
typically only the data parameters need to be stored, instead of the actual data. (Out-
liers may also be stored.) Regression and log-linear models (Section 3.4.5) are examples.
Nonparametric methods for storing reduced representations of the data include his-
tograms (Section 3.4.6), clustering (Section 3.4.7), sampling (Section 3.4.8), and data
cube aggregation (Section 3.4.9).
In data compression, transformations are applied so as to obtain a reduced or “com-
pressed” representation of the original data. If the original data can be reconstructed
from the compressed data without any information loss, the data reduction is called
lossless. If, instead, we can reconstruct only an approximation of the original data, then
the data reduction is called lossy. There are several lossless algorithms for string com-
pression; however, they typically allow only limited data manipulation. Dimensionality
reduction and numerosity reduction techniques can also be considered forms of data
compression.
There are many other ways of organizing methods of data reduction. The computa-
tional time spent on data reduction should not outweigh or “erase” the time saved by
mining on a reduced data set size.
3.4.2
Wavelet Transforms
The discrete wavelet transform (DWT) is a linear signal processing technique that,
when applied to a data vector X, transforms it to a numerically different vector, X , of
wavelet coefficients. The two vectors are of the same length. When applying this tech-
nique to data reduction, we consider each tuple as an n-dimensional data vector, that
is, X = (x
1
, x
2
,
...,x
n
), depicting n measurements made on the tuple from n database
attributes.
3
“How can this technique be useful for data reduction if the wavelet transformed data are
of the same length as the original data?” The usefulness lies in the fact that the wavelet
transformed data can be truncated. A compressed approximation of the data can be
retained by storing only a small fraction of the strongest of the wavelet coefficients.
For example, all wavelet coefficients larger than some user-specified threshold can be
retained. All other coefficients are set to 0. The resulting data representation is therefore
very sparse, so that operations that can take advantage of data sparsity are computa-
tionally very fast if performed in wavelet space. The technique also works to remove
noise without smoothing out the main features of the data, making it effective for data
3
In our notation, any variable representing a vector is shown in bold italic font; measurements depicting
the vector are shown in italic font.