• Data Objects and Attribute Types
  • Basic Statistical Descriptions of Data
  • Data Visualization
  • Measuring Data Similarity and Dissimilarity
  • Summary 

Types of Data Sets

  • Record
    • Relational records
    • Data matrix, e.g., numerical matrix, crosstabs
    • Document data: text documents: term-frequency vector
    • Transaction data
  • Graph and network
    • World Wide Web
    • Social or information networks
    • Molecular Structures
  • Ordered
    • Video data: sequence of images
    • Temporal data: time-series
    • Sequential Data: transaction sequences
    • Genetic sequence data
  • Spatial, image and multimedia:
    • Spatial data: maps
    • Image data:
    • Video data:

Important Characteristics of Structured Data

  • Dimensionality
    • Curse of dimensionality
  • Sparsity
    • Only presence counts
  • Resolution
    • Patterns depend on the scale
  • Distribution
    • Centrality and dispersion

Data Objects

  • Data sets are made up of data objects.
  • A data object represents an entity.
  • Examples:
    • sales database: customers, store items, sales
    • medical database: patients, treatments
    • university database: students, professors, courses
  • Also called samples , examples, instances, data points, objects, tuples.
  • Data objects are described by attributes.
  • Database rows -> data objects; columns ->attributes.


  • Attribute (or dimensions, features, variables): a data field, representing a characteristic or feature of a data object.
    • E.g., customer _ID, name, address
  • Types:
    • Nominal
    • Binary
    • Numeric: quantitative
      • Interval-scaled
      • Ratio-scaled

Attribute Types

  • Nominal: categories, states, or “names of things”
    • Hair_color = {auburn, black, blond, brown, grey, red, white}
    • marital status, occupation, ID numbers, zip codes
  • Binary
    • Nominal attribute with only 2 states (0 and 1)
    • Symmetric binary: both outcomes equally important
      • e.g., gender
    • Asymmetric binary: outcomes not equally important.
      • e.g., medical test (positive vs. negative)
      • Convention: assign 1 to most important outcome (e.g., HIV positive)
  • Ordinal
    • Values have a meaningful order (ranking) but magnitude between successive values is not known.
    • Size = {small, medium, large}, grades, army rankings

Numeric Attribute Types

  • Quantity (integer or real-valued)
  • Interval
      • Measured on a scale of equal-sized units
      • Values have order
        • E.g., temperature in C˚or F˚, calendar dates
      • No true zero-point
  • Ratio
      • Inherent zero-point
      • We can speak of values as being an order of magnitude larger than the unit of measurement (10 K˚ is twice as high as 5 K˚).
        • e.g., temperature in Kelvin, length, counts, monetary quantities

Discrete vs. Continuous Attributes

  • Discrete Attribute
    • Has only a finite or countably infinite set of values
      • E.g., zip codes, profession, or the set of words in a collection of documents
    • Sometimes, represented as integer variables
    • Note: Binary attributes are a special case of discrete attributes
  • Continuous Attribute
    • Has real numbers as attribute values
      • E.g., temperature, height, or weight
    • Practically, real values can only be measured and represented using a finite number of digits
    • Continuous attributes are typically represented as floating-point variables

Basic Statistical Descriptions of Data

  • Motivation
    • To better understand the data: central tendency, variation and spread
  • Data dispersion characteristics
    • median, max, min, quantiles, outliers, variance, etc.
  • Numerical dimensions correspond to sorted intervals
    • Data dispersion: analyzed with multiple granularities of precision
    • Boxplot or quantile analysis on sorted intervals
  • Dispersion analysis on computed measures
    • Folding measures into numerical dimensions
    • Boxplot or quantile analysis on the transformed cube

Measuring the Central Tendency

  •  Mean (algebraic measure) (sample vs. population):  

\[ {\bar{x}}=\frac{1}{n} \sum_{i=1}^{n}x_{i} \]
\[ \mu = \frac{\sum x}{N} \]

    • Note: n is sample size and N is population size.
    • Weighted arithmetic mean:
    • Trimmed mean: chopping extreme values

\[ {\bar{x}}=\frac{\sum_{i=1}^{n}w_{i}x_{i} }{\sum_{i=1}^{n}w_{i}}  \]

  • Median:
    • Middle value if odd number of values, or average of the middle two values otherwise
    • Estimated by interpolation (for grouped data ):  

\[ median = {L_{1}} + (\frac{\frac{n}{2}-(\sum freq)l)}{freq_{median}}) width \] 

Measuring the Central Tendency (cont')

  • Mode
    • Value that occurs most frequently in the data
    • Unimodal, bimodal, trimodal
    • Empirical formula:  
       \[ (mean-mode) = 3 \times (mean-median) \]

Symmetric vs. Skewed Data

  • Median, mean and mode of symmetric, positively and negatively skewed data 

Measuring the Dispersion of Data

  • Quartiles, outliers and boxplots
    • Quartiles: Q1 (25th percentile), Q3 (75th percentile)
    • Inter-quartile range: IQR = Q3 – Q1
    • Five number summary: min, Q1, median, Q3, max
    • Boxplot: ends of the box are the quartiles; median is marked; add whiskers, and plot outliers individually
    • Outlier: usually, a value higher/lower than 1.5 x IQR
  • Variance and standard deviation (sample: s, population: σ)
    • Variance: (algebraic, scalable computation)
\[ {s^{2}}=\frac{1}{n-1}\sum_{i=1}^{n}(x_{i}-\bar{x})^2=\frac{1}{n-1}[\sum_{i=1}^{n}x_{i}^2-\frac{1}{n}(\sum_{i=1}^{n}x_{i})^2] \]

\[ {\sigma ^{2}}=\frac{1}{N}\sum_{i=1}^{n}(x_{i}-\mu)^2=\frac{1}{N}\sum_{i=1}^{n}x_{i}^2-\mu ^2 \]
    • Standard deviation s (or σ) is the square root of variance s2 (or σ2)  

Boxplot Analysis

  • Five-number summary of a distribution
    • Minimum, Q1, Median, Q3, Maximum
  • Boxplot
    • Data is represented with a box
    • The ends of the box are at the first and third quartiles, i.e., the height of the box is IQR
    • The median is marked by a line within the box
    • Whiskers: two lines outside the box extended to Minimum and Maximum
    • Outliers: points beyond a specified outlier threshold, plotted individually

Visualization of Data Dispersion: 3-D Boxplots

Properties of Normal Distribution Curve

  • The normal (distribution) curve
    • From μ–σ to μ+σ: contains about 68% of the measurements (μ: mean, σ: standard deviation)
    • From μ–2σ to μ+2σ: contains about 95% of it
    • From μ–3σ to μ+3σ: contains about 99.7% of it

Graphic Displays of Basic Statistical Descriptions

  • Boxplot: graphic display of five-number summary
  • Histogram: x-axis are values, y-axis repres. frequencies
  • Quantile plot: each value xi is paired with fi indicating that approximately 100 fi % of data are ≤ xi
  • Quantile-quantile (q-q) plot: graphs the quantiles of one univariant distribution against the corresponding quantiles of another
  • Scatter plot: each pair of values is a pair of coordinates and plotted as points in the plane

Histogram Analysis

  • Histogram: Graph display of tabulated frequencies, shown as bars
  • It shows what proportion of cases fall into each of several categories
  • Differs from a bar chart in that it is the area of the bar that denotes the value, not the height as in bar charts, a crucial distinction when the categories are not of uniform width
  • The categories are usually specified as non-overlapping intervals of some variable. The categories (bars) must be adjacent

Histograms Often Tell More than Boxplots

  • The two histograms shown in the left may have the same boxplot representation
    • The same values for: min, Q1, median, Q3, max
  • But they have rather different data distributions

Quantile Plot

  • Displays all of the data (allowing the user to assess both the overall behavior and unusual occurrences)
  • Plots quantile information
    • For a data xi data sorted in increasing order, fi indicates that approximately 100 fi% of the data are below or equal to the value xi

Quantile-Quantile (Q-Q) Plot

  • Graphs the quantiles of one univariate distribution against the corresponding quantiles of another
  • View: Is there is a shift in going from one distribution to another?
  • Example shows unit price of items sold at Branch 1 vs. Branch 2 for each quantile. Unit prices of items sold at Branch 1 tend to be lower than those at Branch 2.

Scatter plot

  • Provides a first look at bivariate data to see clusters of points, outliers, etc
  • Each pair of values is treated as a pair of coordinates and plotted as points in the plane

Positively and Negatively Correlated Data

Uncorrelated Data

Data Visualization

  • Why data visualization?
    • Gain insight into an information space by mapping data onto graphical primitives
    • Provide qualitative overview of large data sets
    • Search for patterns, trends, structure, irregularities, relationships among data
    • Help find interesting regions and suitable parameters for further quantitative analysis
    • Provide a visual proof of computer representations derived
  • Categorization of visualization methods:
    • Pixel-oriented visualization techniques
    • Geometric projection visualization techniques
    • Icon-based visualization techniques
    • Hierarchical visualization techniques
    • Visualizing complex data and relations

Pixel-Oriented Visualization Techniques

  • For a data set of m dimensions, create m windows on the screen, one for each dimension
  • The m dimension values of a record are mapped to m pixels at the corresponding positions in the windows
  • The colors of the pixels reflect the corresponding values

Laying Out Pixels in Circle Segments

  • To save space and show the connections among multiple dimensions, space filling is often done in a circle segment  

Geometric Projection Visualization Techniques

  • Visualization of geometric transformations and projections of the data
  • Methods
    • Direct visualization
    • Scatterplot and scatterplot matrices
    • Landscapes
    • Projection pursuit technique: Help users find meaningful projections of multidimensional data
    • Prosection views
    • Hyperslice
    • Parallel coordinates

Direct Data Visualization

  • Ribbons with Twists Based on Vorticity

Scatterplot Matrices

  • Matrix of scatterplots (x-y-diagrams) of the k-dim. data [total of (k2/2-k) scatterplots]


  • Visualization of the data as perspective landscape
  • The data needs to be transformed into a (possibly artificial) 2D spatial representation which preserves the characteristics of the data 

Parallel Coordinates

  • n equidistant axes which are parallel to one of the screen axes and correspond to the attributes
  • The axes are scaled to the [minimum, maximum]: range of the corresponding attribute
  • Every data item corresponds to a polygonal line which intersects each of the axes at the point which corresponds to the value for the attribute

Parallel Coordinates of a Data Set

Icon-Based Visualization Techniques

  • Visualization of the data values as features of icons
  • Typical visualization methods
    • Chernoff Faces
    • Stick Figures
  • General techniques
    • Shape coding: Use shape to represent certain information encoding
    • Color icons: Use color icons to encode more information
    • Tile bars: Use small icons to represent the relevant feature vectors in document retrieval

Chernoff Faces

  • A way to display variables on a two-dimensional surface, e.g., let x be eyebrow slant, y be eye size, z be nose length, etc.
  • The figure shows faces produced using 10 characteristics--head eccentricity, eye size, eye spacing, eye eccentricity, pupil size, eyebrow slant, nose size, mouth shape, mouth size, and mouth opening): Each assigned one of 10 possible values, generated using Mathematica (S. Dickson)
  • REFERENCE: Gonick, L. and Smith, W. The Cartoon Guide to Statistics. New York: Harper Perennial, p. 212, 1993
  • Weisstein, Eric W. "Chernoff Face." From MathWorld--A Wolfram Web Resource. 

Stick Figure

Hierarchical Visualization Techniques

  • Visualization of the data using a hierarchical partitioning into subspaces
  • Methods
    • Dimensional Stacking
    • Worlds-within-Worlds
    • Tree-Map
    • Cone Trees
    • InfoCube

Dimensional Stacking

  • Partitioning of the n-dimensional attribute space in 2-D subspaces, which are ‘stacked’ into each other
  • Partitioning of the attribute value ranges into classes. The important attributes should be used on the outer levels.
  • Adequate for data with ordinal attributes of low cardinality
  • But, difficult to display more than nine dimensions
  • Important to map dimensions appropriately

Dimensional Stacking

Used by permission of M. Ward, Worcester Polytechnic Institute

  • Visualization of oil mining data with longitude and latitude mapped to the outer x-, y-axes and ore grade and depth mapped to the inner x-, y-axes


  • Assign the function and two most important parameters to innermost world
  • Fix all other parameters at constant values - draw other (1 or 2 or 3 dimensional worlds choosing these as the axes)
  • Software that uses this paradigm
    • N–vision: Dynamic interaction through data glove and stereo displays, including rotation, scaling (inner) and translation (inner/outer) 
    • Auto Visual: Static interaction by means of queries


  • Screen-filling method which uses a hierarchical partitioning of the screen into regions depending on the attribute values
  • The x- and y-dimension of the screen are partitioned alternately according to the attribute values (classes)


  • A 3-D visualization technique where hierarchical information is displayed as nested semi-transparent cubes
  • The outermost cubes correspond to the top level data, while the subnodes or the lower level data are represented as smaller cubes inside the outermost cubes, and so on

Three-D Cone Trees

  • 3D cone tree visualization technique works well for up to a thousand nodes or so
  • First build a 2D circle tree that arranges its nodes in concentric circles centered on the root node
  • Cannot avoid overlaps when projected to 2D
  • G. Robertson, J. Mackinlay, S. Card. “Cone Trees: Animated 3D Visualizations of Hierarchical Information”, ACM SIGCHI'91
  • Graph from Nadeau Software Consulting website: Visualize a social network data set that models the way an infection spreads from one person to the next

Visualizing Complex Data and Relations

  • Visualizing non-numerical data: text and social networks
  • Tag cloud: visualizing user-generated tags
    • The importance of tag is represented by font size/color
  • Besides text data, there are also methods to visualize relationships, such as visualizing social networks

Newsmap: Google News Stories in 2005

Similarity and Dissimilarity

  • Similarity
    • Numerical measure of how alike two data objects are
    • Value is higher when objects are more alike
    • Often falls in the range [0,1]
  • Dissimilarity (e.g., distance)
    • Numerical measure of how different two data objects are
    • Lower when objects are more alike
    • Minimum dissimilarity is often 0
    • Upper limit varies
  • Proximity refers to a similarity or dissimilarity

Data Matrix and Dissimilarity Matrix

  • Data matrix
    • n data points with p dimensions
    • Two modes
  • Dissimilarity matrix
    • n data points, but registers only the distance
    • A triangular matrix
    • Single mode

Proximity Measure for Nominal Attributes

  • Can take 2 or more states, e.g., red, yellow, blue, green (generalization of a binary attribute)
  • Method 1: Simple matching
    • m : # of matches, p : total # of variables

\[ d(i,j)=\frac{p-m}{p} \]

  • Method 2: Use a large number of binary attributes
    • creating a new binary attribute for each of the M nominal states

Proximity Measure for Binary Attributes

  • A contingency table for binary data

  • Distance measure for symmetric binary variables: 
\[ d(i,j)=\frac{r+s}{q+r+s+t} \]
  • Distance measure for asymmetric binary variables: 

\[ d(i,j)=\frac{r+s}{q+r+s} \]

  • Jaccard coefficient (similarity measure for asymmetric binary variables):

\[ sim_{Jaccard}(i,j)=\frac{q}{q+r+s} \]

  • Note: Jaccard coefficient is the same as “coherence”:

\[ coherence(i,j)=\frac{sup(i,j)}{sup(i)+sup(j)-sup(i,j)}=\frac{q}{(q+r)(q+s)-q} \]

Dissimilarity between Binary Variables

  • Example

    • Gender is a symmetric attribute
    • The remaining attributes are asymmetric binary
    • Let the values Y and P be 1, and the value N 0

Standardizing Numeric Data

  • Z-score: 

\[ z=\frac{x-\mu}{\sigma } \]

    • X: raw score to be standardized, μ: mean of the population, σ: standard deviation
    • the distance between the raw score and the population mean in units of the standard deviation
    • negative when the raw score is below the mean, “+” when above
  • An alternative way: Calculate the mean absolute deviation, where

\[ m_{f}= \frac{1}{n}(x_{1f}+x_{2f}+...+x_{nf}) \]

    • standardized measure (z-score):

\[ z_{if}=\frac{(x_{if}-m_{f})}{S_{f}} \]

  • Using mean absolute deviation is more robust than using standard deviation 
\[ s_{f}=\frac{1}{n} (|x_{1f}-m_{f}|+|x_{2f}-m_{f}|+...+|x_{nf}-m_{f}|) \]

Example: Data Matrix and Dissimilarity Matrix

Distance on Numeric Data: Minkowski Distance

  • Minkowski distance: A popular distance measure

where i = (xi1, xi2, …, xip) and j = (xj1, xj2, …, xjp) are two p-dimensional data objects, and h is the order (the distance so defined is also called L-h norm)

  • Properties
    • d(i, j) > 0 if i ≠ j, and d(i, i) = 0 (Positive definiteness)
    • d(i, j) = d(j, i) (Symmetry)
    • d(i, j) ≤ d(i, k) + d(k, j) (Triangle Inequality)
  • A distance that satisfies these properties is a metric

Special Cases of Minkowski Distance

  • h = 1: Manhattan (city block, L1 norm) distance
    • E.g., the Hamming distance: the number of bits that are different between two binary vectors
\[ d(i,j)=|x_{i1}-x_{j1}|+|x_{i2}-x_{j2}|+...+|x_{ip}-x_{jp}| \]

  • h = 2: (L2 norm) Euclidean distance

\[ d(i,j)=\sqrt{(|x_{i1}-x_{j1}|^2+|x_{i2}-x_{j2}|^2+...+|x_{ip}-x_{jp}|^2)} \]

  • h →≈ . “supremum” (Lmax norm, L norm) distance.
    • This is the maximum difference between any component (attribute) of the vectors
\[ d(i,j)=\lim_{h\rightarrow \infty }(\sum_{f=1}^{p}|x_{if}-x_{jf}|^{h})^\frac{1}{h} =max_{f}^{p}|x_{if}-x_{jf}| \]

Example: Minkowski Distance

Ordinal Variables

  • An ordinal variable can be discrete or continuous
  • Order is important, e.g., rank
  • Can be treated like interval-scaled
    • replace xif by their rank 

\[ r_{if} \epsilon \left \{ 1,...,M_{f} \right \} \]

    • map the range of each variable onto [0, 1] by replacing i-th object in the f-th variable by
    • compute the dissimilarity using methods for interval-scaled variables
 \[ z_{if} = \frac{r_{if}-1}{M_{f}-1} \]

Attributes of Mixed Type

  • A database may contain all attribute types
    • Nominal, symmetric binary, asymmetric binary, numeric, ordinal
  • One may use a weighted formula to combine their effects

\[ d(i,j) = \frac{\sum_{f=1}^{p} \delta _{ij}^{(f)} d_{ij}^{(f)}}{\sum_{f=1}^{p} \delta _{ij}^{(f)}} \]

    • f is binary or nominal:
      • dij(f) = 0 if xif = xjf , or dij(f) = 1 otherwise
    • f is numeric: use the normalized distance
    • f is ordinal
      • Compute ranks rif and
      • Treat zif as interval-scaled
\[ z_{if} = \frac{r_{if}-1}{M_{f}-1} \]

Cosine Similarity

  • A document can be represented by thousands of attributes, each recording the frequency of a particular word (such as keywords) or phrase in the document.

  • Other vector objects: gene features in micro-arrays, …
  • Applications: information retrieval, biologic taxonomy, gene feature mapping, ...
  • Cosine measure: If d1 and d2 are two vectors (e.g., term-frequency vectors), then
                 cos(d1, d2) =  (d1 ⋅ d2) /||d1|| ||d2|| ,
       where ⋅ indicates vector dot product, ||d||: the length of vector d

Example: Cosine Similarity

  • cos(d1, d2) = (d1d2) / ||d1|| ||d2|| ,
    where ⋅ indicates vector dot product, ||d|: the length of vector d
  • Ex: Find the similarity between documents 1 and 2.
    d1 = (5, 0, 3, 0, 2, 0, 0, 2, 0, 0)
    d2 = (3, 0, 2, 0, 1, 1, 0, 1, 0, 1)
    d1 d2 = 5*3+0*0+3*2+0*0+2*1+0*1+0*1+2*1+0*0+0*1 = 25
    ||d1|| = (5*5+0*0+3*3+0*0+2*2+0*0+0*0+2*2+0*0+0*0)0.5 = (42) 0.5 = 6.481
    ||d2|| = (3*3+0*0+2*2+0*0+1*1+1*1+0*0+1*1+0*0+1*1)0.5 = (17) 0.5 = 4.12
    cos(d1, d2 ) = 0.94


  • Data attribute types: nominal, binary, ordinal, interval-scaled, ratio-scaled
  • Many types of data sets, e.g., numerical, text, graph, Web, image.
  • Gain insight into the data by:
    • Basic statistical data description: central tendency, dispersion, graphical displays
    • Data visualization: map data onto graphical primitives
    • Measure data similarity
  • Above steps are the beginning of data preprocessing.
  • Many methods have been developed but still an active area of research.


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