Chapter 2: Displaying Data

Effective graphs are the principal means of

communicating results to a wider audience

Characteristics of a good graph

• Show the Data

• Make patterns in the data easy to see

• Represent magnitudes honestly

• Draw graphical elements clearly

• All figures should have captions

• Be sure that figures with color are effective in black and white

Frequency table

It is used to display a frequency distribution for categorical or numerical data

One Variable Categorical Graph

Bar Graph

One Variable Numerical Graph

Histogram

Contingency Tables

describe the association between two (or more) categorical variables by displaying frequencies of all combinations of categories

Two Numerical Variable Graph

Scatter Plot

Two Categorical Variables

• Grouped bar graph

• Mosaic plot

One numerical variable and one categorical variable graph

• Strip Chart

• Violin Plot

• Multiple Histograms

• Cumulative frequency distribution

Outliers

Extreme data points lying well away from the rest of the data

Why do we use numerical displays

• Because it’s easy to spot patterns

• Helps to spot outliers

• Humans are visual

• A picture is worth a thousand words

Waffle Plot

Useful when looking at percentage as a whole

What should the data-to-ink ratio be?

There should be a high data-to-ink ratio

Why are pie charts and 3D charts bad?

• Hard to make accurate interpretations

• Perspectives skews visual perception

Common Errors When Making Graphs

• Truncation of the y-axis

• Cumulative graphs

• Ignoring conventions

• Axes are mislabelled or missing