Chapter 4
Displaying Quantitative Data
AP Statistics
Displaying Quantitative Data
• Histogram
• Stem-and-Leaf Plots
• Dotplots
• (Timeplots)
Histograms
• Bins and counts give the
distribution of the
quantitative data
• Bars touch—data is
continuous
• Relative frequency
histogram—useful and
shows percentages, not
counts
Stem-and-Leaf Plot
• Can see each individual
data point
• Stem is like bin
• Might need to “split”
3
4
5
6
7
34779
6677789
356777777
0001
99
2
2
3
3
4
4
022222
577799999
44444
667789999
2333344444
577779
Key: 3 4 = 34
Key: 2 4 = 24
Dotplot
• Useful in seeing how
many individual data
points in bin
• Good for small sets of
data
• Not used too often
Create Histogram and Stem-and-Leaf
Plots of Presidential Ages at
Inauguration
* Always Check the Quantitative Data
Condition!!!
Describing a Distribution
• Whenever you are describing a distribution
you need to describe it by the
– Shape
– Center
– Spread
– Any Unusual points (outliers, gaps)
• We will be go into more detail in Chapter 5
Shape
• Is the shape?
• Uniform, Symmetric,
Skewed
• How many modes (high
points)
– Unimodal, bimodal,
multimodal
Center
• Where is the data
“center”
• Is vague for right now—
more specific in Chapter
5
• More difficult if the
distribution is not
symmetric
Spread
• How spread out is the
data
• For now—”large”,
“small”, “values
between”
• Very vague—more
specific in Chapter 5
Unusual Points
• Outliers
• Gaps
• Sometimes these are
the most important
aspects of the data
Describe the Presidential Data
Describe the Obesity Rate Data
Timeplots
• Used to see trends and
patterns over time
• What you might
consider line graph
Reexpressing data
• Skewed data is hard to summarize
• Will “make” data symmetric
• This will help better fine center and spread
– If skewed right: use logs or square roots
– If skewed left: use squares