What is a Dotplot?

A dotplot is a type of graphic display used to compare frequency counts within categories or groups.

Dotplot Overview

As you might guess, a dotplot is made up of dots plotted on a graph. Here is how to interpret a dotplot.

• Each dot represents a specific number of observations from a set of data. (Unless otherwise indicated, assume that each dot represents one observation. If a dot represents more than one observation, that should be explicitly noted on the plot.)
• The dots are stacked in a column over a category, so that the height of the column represents the relative or absolute frequency of observations in the category.
• The pattern of data in a dotplot can be described in terms of symmetry and skewness only if the categories are quantitative. If the categories are qualitative (as they often are), a dotplot cannot be described in those terms.

Compared to other types of graphic display, dotplots are used most often to plot frequency counts within a small number of categories, usually with small sets of data.

Dotplot Example

Here is an example to show what a dotplot looks like and how to interpret it. Suppose 30 first graders are asked to pick their favorite color. Their choices can be summarized in a dotplot, as shown below.

Each dot represents one student, and the number of dots in a column represents the number of first graders who selected the color associated with that column. For example, Red was the most popular color (selected by 9 students), followed by Blue (selected by 7 students). Selected by only 1 student, Indigo was the least popular color.

In this example, note that the category (color) is a qualitative variable; so it is not appropriate to talk about the symmetry or skewness of this dotplot. The dotplot in the next section uses a quantitative variable, so we will illustrate skewness and symmetry of dotplots in the next section.