2: Graphing Distributions


Graphing data is the first and often most important step in data analysis. In this day of computers, researchers all too often see only the results of complex computer analyses without ever taking a close look at the data themselves. This is all the more unfortunate because computers can create many types of graphs quickly and easily. This chapter covers some classic types of graphs such as bar charts that were invented by William Playfair in the 18th century as well as graphs such as box plots invented by John Tukey in the 20th century.

• 2.1: Graphing Qualitative Variables
This section examines graphical methods for displaying the results of the interviews. We’ll learn some general lessons about how to graph data that fall into a small number of categories. A later section will consider how to graph numerical data in which each observation is represented by a number in some range. The key point about the qualitative data that occupy us in the present section is that they do not come with a pre-established ordering (the way numbers are ordered).
• 2.2: Quantitative Variables
Quantitative variables are variables measured on a numeric scale. Height, weight, response time, subjective rating of pain, temperature, and score on an exam are all examples of quantitative variables. Quantitative variables are distinguished from categorical (sometimes called qualitative) variables such as favorite color, religion, city of birth, and favorite sport in which there is no ordering or measuring involved.
• 2.3: Stem and Leaf Displays
A stem and leaf display is a graphical method of displaying data. It is particularly useful when your data are not too numerous. In this section, we will explain how to construct and interpret this kind of graph.
• 2.4: Histograms
A histogram is a graphical method for displaying the shape of a distribution. It is particularly useful when there are a large number of observations.
• 2.5: Frequency Polygons
Frequency polygons are a graphical device for understanding the shapes of distributions. They serve the same purpose as histograms, but are especially helpful for comparing sets of data. Frequency polygons are also a good choice for displaying cumulative frequency distributions.
• 2.6: Box Plots
Box plots are useful for identifying outliers and for comparing distributions.
• 2.7: Box Plot Demo
• 2.8: Bar Charts
Bar charts can be used to present other kinds of quantitative information, not just frequency counts in histograms.
• 2.9: Line Graphs
A line graph is a bar graph with the tops of the bars represented by points joined by lines (the rest of the bar is suppressed).
• 2.10: Dot Plots
Dot plots can be used to display various types of information.
• 2.11: Statistical Literacy
• 2.E: Graphing Distributions (Exercises)