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2.5
Summary
Data sets are made up of data objects. A data object represents an entity. Data objects
are described by attributes. Attributes can be nominal, binary, ordinal, or numeric.
The values of a nominal (or categorical) attribute are symbols or names of things,
where each value represents some kind of category, code, or state.
Binary attributes are nominal attributes with only two possible states (such as 1 and
0 or true and false). If the two states are equally important, the attribute is symmetric;
otherwise it is asymmetric.
An ordinal attribute is an attribute with possible values that have a meaningful order
or ranking among them, but the magnitude between successive values is not known.
A numeric attribute is quantitative (i.e., it is a measurable quantity) represented
in integer or real values. Numeric attribute types can be interval-scaled or ratio-
scaled. The values of an interval-scaled attribute are measured in fixed and equal
units. Ratio-scaled attributes are numeric attributes with an inherent zero-point.
Measurements are ratio-scaled in that we can speak of values as being an order of
magnitude larger than the unit of measurement.
Basic statistical descriptions provide the analytical foundation for data preprocess-
ing. The basic statistical measures for data summarization include mean, weighted
mean, median, and
mode for measuring the central tendency of data; and
range, quan-
tiles, quartiles, interquartile range, variance, and
standard deviation for measuring the
dispersion of data. Graphical representations (e.g., boxplots, quantile plots, quantile–
quantile plots, histograms, and
scatter plots) facilitate visual inspection of the data and
are thus useful for data preprocessing and mining.
Data visualization techniques may be
pixel-oriented, geometric-based, icon-based, or
hierarchical. These methods apply to multidimensional relational data. Additional
techniques have been proposed for the visualization of complex data, such as text
and social networks.
Measures of object similarity and dissimilarity are used in data mining applications
such as clustering, outlier analysis, and nearest-neighbor classification. Such mea-
sures of proximity can be computed for each attribute type studied in this chapter,
or for combinations of such attributes. Examples include the Jaccard coefficient for
asymmetric binary attributes and Euclidean, Manhattan, Minkowski, and supremum
distances for numeric attributes. For applications involving sparse numeric data vec-
tors, such as term-frequency vectors, the cosine measure and the Tanimoto coefficient
are often used in the assessment of similarity.
2.6
Exercises
2.1 Give three additional commonly used statistical measures that are not already illus-
trated in this chapter for the characterization of data dispersion. Discuss how they can
be computed efficiently in large databases.
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2.2 Suppose that the data for analysis includes the attribute
age. The
age values for the data
tuples are (in increasing order) 13, 15, 16, 16, 19, 20, 20, 21, 22, 22, 25, 25, 25, 25, 30,
33, 33, 35, 35, 35, 35, 36, 40, 45, 46, 52, 70.
(a) What is the mean of the data? What is the median?
(b) What is the mode of the data? Comment on the data’s modality (i.e., bimodal,
trimodal, etc.).
(c) What is the midrange of the data?
(d) Can you find (roughly) the first quartile (Q
1
) and the third quartile (Q
3
) of the data?
(e) Give the five-number summary of the data.
(f) Show a boxplot of the data.
(g) How is a quantile–quantile plot different from a quantile plot?
2.3 Suppose that the values for a given set of data are grouped into intervals. The intervals
and corresponding frequencies are as follows:
age
frequency
1–5
200
6–15
450
16–20
300
21–50
1500
51–80
700
81–110
44
Compute an approximate median value for the data.
2.4 Suppose that a hospital tested the age and body fat data for 18 randomly selected adults
with the following results:
age
23
23
27
27
39
41
47
49
50
%fat
9.5
26.5
7.8
17.8
31.4
25.9
27.4
27.2
31.2
age
52
54
54
56
57
58
58
60
61
%fat
34.6
42.5
28.8
33.4
30.2
34.1
32.9
41.2
35.7
(a) Calculate the mean, median, and standard deviation of
age and
%fat.
(b) Draw the boxplots for age and %fat.
(c) Draw a scatter plot and a q-q plot based on these two variables.
2.5 Briefly outline how to compute the dissimilarity between objects described by the
following:
(a) Nominal attributes
(b) Asymmetric binary attributes