Compute Q1, Q2 and Q3. Also, compute the interquartile range
IQR = Q3 - Q1. Example: Suppose that the
dataset consists of these hypothetical test scores:
>5 39 75 79 85
90 91 93 93 98
Q1 = 75, Q2 = 87.5, Q3 = 93. IQR = 93 - 75 = 18.
Draw three horizontal lines, all of the same length and
all starting at the same x-value: one at height Q1, the second at Q2 and
the third at Q3. Example:Boxplot after Step 2.
Draw two vertical lines, one at connecting the left endpoints
of the lines and the other connecting their right endpoints. Example:Updated boxplot after Step 3.
Compute the inner fences IF1 = Q1 - 1.5 * IQR and
IF2 = Q3 + 1.5 * IQR. Example: The inner fences are
Draw a whisker downward from Q1 to IF1 or Q0, whichever comes first.
Draw a whisker upward from Q3 to IF2 or Q4, whichever comes first. Example:Boxplot after adding the whiskers in Step 4.
Compute the outer fences OF1 = Q1 - 3 * IQR and OF2 = Q3 + 3 * IQR. Example: The outer fences are
Extreme outliers are observations that are beyond one of the
outer fences OF1 or OF2. Mark any extreme outliers on the boxplot
with an asterisk (*). Example: The only observation less than
OF1 = 21 is 5. Boxplot after marking 5 with a *.
Mild outliers are observations that are between an inner and
outer fence. Mild outliers are marked with a circle (O). Example: The only observation that is
between an inner fence and an outer fence is 39, which is between
IF1 = 48 and OF1 = 21. Boxplot after marking 39 with a O.
Compare your boxplot with one constructed by R from the same data.
Mild vs. Extreme Outliers
Extreme outliers are data points that are more extreme
than Q1 - 3 * IQR or Q3 + 3 * IQR.
Extreme outliers are outside of either outer fence.
Extreme outliers are marked with an asterisk (*) on the
boxplot.
Mild outliers are data points that are more extreme than
than Q1 - 1.5 * IQR or Q3 + 1.5 * IQR, but are not extreme outliers.
Mild outliers are outside of either inner fence, but not outside
of any outer fence.
Mild outliers are marked with a circle (O) on the boxplot.