Displaying Quantitative Data

1
Chapter 4

• Displaying Quantitative Data

2
Describing One Quantitative Variable

• Distribution of variable
• Summary of different values observed for the
  variable
• Make a picture
• Histogram
• Stem-and-Leaf Display
• Boxplot (next chapter)

3
Histogram of Percent of Population of Hispanic
Origin
4
Stem-and-Leaf Display

• Picture of Distribution.
• Generally used for smaller data sets.
• Group data like histograms.
• Still have original values (or close to it).

5
Stem-and-Leaf Display

• Two columns
• Left column Stem
• Right column Leaf
• Leaf
• Contains the last digit of the values.
• Arranged in increasing order away from stem.
• Stem
• Contains the rest of the values.
• Usually arranged in increasing order from top to
  bottom.
• JMP does opposite, increasing order from bottom
  to top!

6
Stem and Leaf Plot of Percent of Population of
Hispanic Origin
7
Comparing Two Related Variables

• Histograms
• Make sure bins scaling for each histogram is the
  same.

8
Histograms of Female and Male Heights
9
Looking at Distributions

• Shape
• How many humps (called modes)?
• None uniform
• One unimodal
• Two bimodal
• Three or more multimodal

10
Looking at Distributions

• Shape
• Is it symmetric?
• Symmetric roughly equal on both sides
• Skewed more values on one side
• Skewed Right Tail stretches to large values
• Skewed Left Tail stretches to small values
• Skewed not screwed

11
Looking at Distributions

• Shape
• Are there any outliers?
• Interesting observations in data
• Can impact statistical methods

12
Looking at Distributions

• Center
• A single number to describe the data.
• Calculate different numbers for center. Right
  now, just
• EYE BALL IT

13
Looking at Distributions

• Spread
• Variation in the values
• Smallest observation
• Largest observation
• Take into account any outliers.