Values which are "far" from the middle of the distribution are referred to as
outliers and extreme values if they meet certain conditions.
A data point is deemed to be an extreme value if the following conditions hold:
data point value > UBV + 2*o.c.*(UBV - LBV)
or
data point value < LBV - 2*o.c.*(UBV - LBV)
where
UBV is the upper value of the box in the box plot (e.g., the mean + standard error or the 75th percentile).
LBV is the lower value of the box in the box plot (e.g., the mean - standard error or the 25th percentile).
o.c. is the
outlier coefficient (when this coefficient equals 1.5, the extreme values are those which are outside the 3 box length range from the upper and lower value of the box).
For example, the following diagram illustrates the ranges of
outliers and extremes in the "classic"
box and whisker plot (for more information about box plots, see Tukey, 1977).
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