Page 133 - Statistics for Dummies

Chapter 7: Going by the Numbers: Graphing Numerical Data
*
**
*
*
*
*
Figure 7-7:
Boxplot
of Best
Actress
ages (1928–
40
30
60
70
50
2009; n = 83 20 Age of Best Actress Award Winners 1928–2009 (n = 83) 80 117
actresses). Age

Interpreting a boxplot
Similar to a histogram (see the section “Interpreting a histogram”), a box-
plot can give you information regarding the shape, center, and variability of
a data set. Boxplots differ from histograms in terms of their strengths and
weaknesses, as you see in the upcoming sections, but one of their biggest
strengths is how they handle skewed data.
Checking the shape with caution!
A boxplot can show whether a data set is symmetric (roughly the same on
each side when cut down the middle) or skewed (lopsided). A symmetric
data set shows the median roughly in the middle of the box. Skewed data
show a lopsided boxplot, where the median cuts the box into two unequal
pieces. If the longer part of the box is to the right (or above) the median, the
data is said to be skewed right. If the longer part is to the left (or below) the
median, the data is skewed left.

3/25/11 8:16 PM
12_9780470911082-ch07.indd 117 3/25/11 8:16 PM
12_9780470911082-ch07.indd 117