1. Boxplots

2. Why use boxplots? ease of construction convenient handling of outliers
construction is not subjective (like histograms)
Used with medium or large size data sets (n > 10)
useful for comparative displays

3. Disadvantage of boxplots
does not retain the individual observations
should not be used with small data sets (n < 10)

4. Modified boxplots display outliers
fences mark off mild & extreme outliers
whiskers extend to largest (smallest) data value inside the fence

5. Interquartile Range (IQR) – is the range (length) of the box
Inner fence
Interquartile Range (IQR) – is the range (length) of the box
Q3 - Q1
Q1 – 1.5IQR
Q IQR
Any observation outside this fence is an outlier! Put a dot for the outliers.

6. Modified Boxplot . . .
Draw the “whisker” from the quartiles to the observation that is within the fence!

7. The five-number summary: min, max, Q1, Q3 and median
Maximum
value
Minimum value
IQR=Q3-Q1 is called interquartile range

8. A report from the U.S. Department of Justice gave the following percent increase in federal prison populations in 20 northeastern & mid-western states in 1999.
Create a box plot with outliers.

9. Enter data in the spreadsheet on your calculator

10. Box plot with outliers Q1=4.45 Q3=6.35 Median=5.4 IQR=Q3-Q1=1.9
1.5 times 1.9=2.85
Q3+2.85=9.2
Q1-2.85=1.6
Outliers: outside interval[1.6,9.2]

11. Box plots for data with frequencies

12. Ctrl Menu on the x-axis

13. Boxplot
No outliers.

14. One-variable statistics in lists