Consider the data set shown on the dot plot. Determine the median of the data set and select the column of the dot plot in which the median is located.

Answer:A dot plot displays the distribution of a continuous variable. Consider Example 3.9 following where we want to compare the distribution of the continuous variable, systolic blood pressure, across a nominal variable such as age grouped into two categories - under 60 years of age and 60 years and over. These plots give a visual comparison of the center of the observations as well as providing some idea about how the observations vary. Like stem-and-leaf plots, dot plots are used for a relatively small data set.

Example 3.9

Dot plots comparing SBP across the age groups of "<60" and "≥60" are shown in Figure 3.11.

Figure 3.11. Dot plots for systolic blood pressure by age group, DIG40.

The dot plots allow us to see the data in its entirety. From the graphs, we see that the largest systolic blood pressure observation in the 60 and over group is considerably larger than the corresponding largest value in the under 60 years of age group. Also notice that dots are stacked up for observations with the same measurement value. For example, the stacked dots make it clear that there are two observations with the systolic blood pressure reading of 170 mmHg.