What is a Dotplot?
A dotplot is a type of graphic display used to compare frequency counts within categories or groups.
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Dotplot Overview
As you might guess, a dotplot is made up of dots plotted on a graph.
Here is how to interpret a dotplot.
- 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.
* * * * * * * * * | * * | * * * | * * * * * | * * * * * * * | * | * * * |
Red | Orange | Yellow | Green | Blue | Indigo | Violet |
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.
Test Your Understanding
Problem 1
The dotplot below shows the number of televisions owned by each family
on a city block.
* | * * * | * * * * * | * * * * | * * * | * * | * | * | * |
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
Which of the following statements are true?
(A) The distribution is right-skewed with no outliers.
(B) The distribution is right-skewed with one outlier.
(C) The distribution is left-skewed with no outliers.
(D) The distribution is left-skewed with one outlier.
(E) The distribution is symmetric.
Solution
The correct answer is (A). Most of the observations are on the
left side of the distribution, so the distribution is
right-skewed.
And none of the observations is extreme, so
there are no
outliers.
Note: Because the categories are quantitative
(i.e., numbers), it is appropriate to describe the skewness of
the data in this dotplot.