Page 137 - Statistics for Dummies

Time between Eruptions of Old Faithful (n = 222)
90
80
70
Figure 7-11:
Boxplot of
60
eruption
times for
50
Old Faithful
geyser
40
(n = 222).
Time (minutes) 100 Chapter 7: Going by the Numbers: Graphing Numerical Data 121
Denoting outliers
Looking at the boxplot in Figure 7-7 for the Best Actress ages data, you see
a set of outliers (seven in all) on the right side of the data set, marked by a
group of stars (as described in Step 8 in the earlier section “Making a box-
plot”). Three of the stars lie on top of one another because three actresses
were the same age, 61, when they won their Oscars.
You verify these outliers by applying the rule described in Step 6 of the sec-
tion “Making a Boxplot.” The IQR is 11 (from Figure 7-3), so you take 11 ∗ 1.5 =
16.5 years. Add this amount to Q and you get 39 + 16.5 = 55.5 years; subtract-
3
ing this amount from Q you get 28 – 16.5 = 11.5 years. So an actress whose
1
age was below 11.5 years (that is, 11 years old and under) or above 55.5 years
(that is, 56 years old or over) is considered to be an outlier.
Of course, the lower end of this boundary (11.5 years) isn’t relevant because
the youngest actress was 21 (Figure 7-3 shows the minimum is 21). So you
know there aren’t any outliers on the low end of this data set.
However, seven outliers are on the high end of the data set, where the
56-and-over actresses’ ages are. Table 7-2 shows the information on all seven
outliers in the Best Actress ages data set.

3/25/11 8:16 PM
12_9780470911082-ch07.indd 121 3/25/11 8:16 PM
12_9780470911082-ch07.indd 121