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SecTIoN 1c Displaying Quantitative Data with Graphs 23

HOW TO MAKE A DOTPLOT

1. Draw and label the axis. Draw a horizontal axis and put the name of
the quantitative variable underneath it. Be sure to include units of mea-
surement if appropriate.
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2. Scale the axis. Find the smallest and largest values in the data set. Start
the horizontal axis at a convenient number equal to or less than the
smallest value and place tick marks at equal intervals until you equal or
exceed the largest value.
3. Plot the values. Mark a dot above the location on the horizontal axis
corresponding to each data value. Try to make all the dots the same size
and space them out equally as you stack them.

Remember what we said in Section 1B: making a graph is not an end in itself.
When you look at a graph, always ask, “What do I see?” The dotplot in Figure
1.2 shows that 8/71 = 0.113 = 11.3% of the Flint water samples had lead levels

that exceeded 15 parts per billion. Does that mean Flint’s water system required
action based on the EPA guideline? We’ll answer that question — and tell you the
rest of the story about the Flint water crisis — in Section 1E.
EXAMPLE eating healthy Skills 2.A, 2.B
Displaying quantitative data: Dotplots

PROBLEM: How healthy is plant-based yogurt? The Nutrition

Action Healthletter provided data on calories, saturated fat, protein,
calcium, and total sugars for many popular brands of yogurt. Here
are the data on the amount of added sugar, in teaspoons (tsp), in
single-serving containers for all of the plant-based yogurt brands: 30

0.0 1.0 1.0 2.5 0.0 0.0 1.0 1.5 2.0 2.0 3.0 3.5 2.5 Clare Gainey/Alamy Stock Photo
3.0 5.0 3.0 3.5 3.5 3.5 2.0 4.0 3.5 2.0 3.5 1.5 2.0
(a) Make a dotplot of these data.
(b) The American Heart Association (AHA) recommends a maximum of 6 teaspoons of added sugar per day

31
for women. What percentage of these plant-based yogurt brands have less than half of the AHA’s daily
recommendation?
SOLUTION:
(a)
To make the dotplot:
1. Draw and label the axis. Be sure to include units
along with the variable name.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 2. Scale the axis. The smallest value is 0.0 and the
Amount of added sugar (tsp) largest value is 5.0. So we choose a scale from 0
to 5 with tick marks 0.5 unit apart.

( b) Half of the AHA’s recommended daily maximum would 3. Plot the values.

be 3 teaspoons of added sugar, and 15/26 = 0.577 =
57.7% of these plant-based yogurt brands have less than
3 teaspoons of added sugar.
FoR PRAcTIce, TRY eXeRcISe 3

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