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A dot plot is one of the simplest graphs. To construct this graph, the value of each observation is plotted on a real line. It provides visual information about the distribution of a single variable. For illustration, we consider the following example.

Example 2.4.1 (Defective motors) The following data give the number of defective motors received in 20 different shipments:

8	12	10	16	10	25	21	15	17	5
26	21	29	8	6	21	10	17	15	13

Construct a dot plot for these data.

Solution: To construct a dot plot, draw a horizontal line with its scale beginning with a number less than the smallest observation and ending at a number greater than the largest observation. Then, construct the dot plot by plotting the given data points as shown in Figure 2.4.1.

Figure 2.4.1 Dot plot for the data on defective motors received in 20 different shipments.

Dot plots are more useful when the sample size is small. A dot plot gives us, for example, information about how the data are scattered and where most of the observations are concentrated. For instance, in this example, we see that the minimum number of defective motors and the maximum number of defective motors received in any shipment were 5 and 29, respectively. Also, we can see that 75% of the time, the number of defective motors was between 8 and 21 (inclusive) for these shipments, and so on.