Envisioning Information

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Envisioning Information

• Lecture 2
• Simple Graphs and Charts

Ken Brodlie School of Computing University of
Leeds
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Lecture Outline

• Preliminaries
• Definitions
• Datatypes
• Simple Data Presentation
• Graphs and charts

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Fundamentals

• Basic Datatypes correspond to different levels of
  measurement
• Data can be
• Categorical - labels
• Numerical numbers

• Categorical
• Nominal
• No sense of order
• Apples, oranges,
• Ordinal
• Ordered in sequence
• January, February, ..
• Numerical
• Continuous
• Real numbers
• Height of students in class
• Discrete
• Typically whole numbers
• Marks in an exam

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Question

• Give an example for each class in which numbers
  are involved

• Categorical - nominal
• Categorical - ordinal
• Numerical continuous
• Numerical - discrete

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Exploratory Data Analysis

• Two stage process
• Exploratory Search for evidence using all tools
  available
• Confirmatory evaluate strength of evidence using
  classical data analysis

• Pioneering figure is John Tukey
• New approach to data analysis, heavily based on
  visualization, as an alternative to classical
  data analysis
• See wikipedia

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Simple Data Presentation
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Simple Data Presentation

• Simple data tables are often presented as line
  graphs, bar graphs, pie charts, dot graphs,
  histograms
• Which should we use and when?

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Line Graph

• Fundamental technique of data presentation
• Used to compare two variables
• X-axis is often the control variable
• Y-axis is the response variable
• Good at
• Showing specific values
• Trends
• Trends in groups (using multiple line graphs)

Students participating in sporting activities
Any critical comments here?
Note graph labelling is fundamental
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Simple Representations Bar Graph

• Bar graph
• Presents categorical variables
• Height of bar indicates value
• Double bar graph allows comparison
• Note spacing between bars
• Can be horizontal (when would you use this?)

Number of police officers
Internet use at a school
Note more space for labels
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Dot Graph

• Very simple but effective
• Horizontal to give more space for labelling

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Pie Chart

• Pie chart summarises a set of categorical/nominal
  data
• But use with care
• too many segments are harder to compare than in
  a bar chart

Should we have a long lecture?
Favourite movie genres
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Histograms

• Histograms summarise discrete or continuous data
  that are measured on an interval scale
• No gaps if variable is continuous

Distribution of salaries in a company
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Scatter Plot

• Used to present measurements of two variables
• Effective if a relationship exists between the
  two variables

Car ownership by household income
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Scatter Plots in Excel

• The scatter plot is a fundamental tool in Excel
• Chart type XY (Scatter) and subtype Unconnected
  Points

http//www2.ncsu.edu8010/ncsu/chemistry/resource/
excel/excel.html
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Regression Line

• Excel allows you to add a linear regression line
  (trend line)

Remember correlation does not imply causality
ie a relationship exists but one is not
necessarily causing the other there may be
a third factor?
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Tukey Sum-Difference Plot
Better understanding of residuals
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Box Plots

• In some situations we have, not a single data
  value at a point, but a number of data values, or
  even a probability distribution
• When might this occur?
• Tukey proposed the idea of a boxplot to visualize
  the distribution of values
• For explanation and some history, see
• http//mathworld.wolfram.com/Box-and-WhiskerPlot.h
  tml
• http//en.wikipedia.org/wiki/Box_plot

M median Q1, Q3 quarrtiles Whiskers 1.5
interquartile range Dots - outliers
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Acknowledgement

• Thanks to Statistics Canada an excellent web
  site for simple data presentation
• http//www.statcan.ca/english/edu/power/toc/conten
  ts.htm

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Exercise for next week

• Understand a bit more about the merits of pie
  charts and bar graphs
• Create a dataset with roughly equal numbers in
  each class
• Which is best if the task is to discriminate?

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Exercise for next week

• Over the next week look for examples of basic
  graphs
• In newspapers, magazines or other print media
• On news web sites or other electronic media
• Analyse two examples
• One should be a example where you think the use
  of graphics is good
• One should be bad
• Be ready next week to present these results to
  the class

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Envisioning Information Practical Work

• Gnuplot
• R
• Excel