Outliers and Extremes

When creating box plots (either 2D Box Plots or 2D Scatterplots with Box Plots), you can select to display none (select Off), only Outliers, only Extreme values, or both Outliers & Extremes in the box plot. These options are available in the Outliers group box on the respective Advanced tabs.

Outliers and Extremes

Values that are far from the middle of the distribution are referred to as outliers and extreme values if they meet the conditions specified below.

Outliers

For Graphs menu box plots, a data point is deemed to be an outlier if the following conditions hold:

data point value > UBV + o.c.*(UBV - LBV)

or

data point value < LBV - o.c.*(UBV - LBV)

where:

UBV is the upper value of the box in the box plot ( the mean + std.err. or the 75th percentile).
LBV is the lower value of the box in the box plot (the mean - std.err. or the 25th percentile).
o.c. is the outlier coefficient specified in the Outliers option on the 2D Box Plots - Advanced tab (by default, the outlier coefficient is 1.5; for example, see 2D Box Plots)

Non-outlier range

See Non-outlier range.

Extremes

For Graphs menu box plots, 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 (the mean + std.err. or the 75th percentile).
LBV is the lower value of the box in the box plot (the mean - std.err. or the 25th percentile).
o.c. is the outlier coefficient specified in the Outliers option on the 2D Box Plots - Advanced tab (by default, the outlier coefficient is 1.5, thus, the extreme values are those which are outside the 3 box length range from the upper and lower value of the box; for example, see 2D Box Plots)

The following diagram illustrates the ranges of outliers and extremes in the classic box and whisker plot where the upper box value (UBV) is the 75th percentile, the lower box value (LBV) is the 25th percentile, and the outlier coefficient is equal to 1.5.