The Box-and-whisker plot (Tukey, 1977) displays a graphical statistical summary of a variable. This plot can also be useful to detect outliers.
- In the basic Box-and-whisker plot, the central box represents the values from the lower to upper quartile (25 to 75 percentile). The middle line represents the median. The horizontal line extends from the minimum to the maximum value, excluding "outside" and "far out" values which are displayed as separate points.
- An outside value is defined as a value that is smaller than the lower quartile minus 1.5 times the interquartile range, or larger than the upper quartile plus 1.5 times the interquartile range (inner fences).
- A far out value is defined as a value that is smaller than the lower quartile minus 3 times the interquartile range, or larger than the upper quartile plus 3 times the interquartile range (outer fences). These values are plotted with a different marker in a different color.
- Select the variable of interest.
- Optionally select a filter to include a subset of cases.
- If the data require a logarithmic transformation (e.g. when the data are positively skewed), select the Logarithmic transformation option.
- Plot all data: includes all the data points in the graph. This option is very useful since in this case the graph has the advantage not to conceal the real data.
- Graph orientation: optional vertical orientation of the box-and-whisker plot.
- Altman DG (1991) Practical statistics for medical research. London: Chapman and Hall.
- Tukey JW (1977) Exploratory data analysis. Reading, Mass: Addison-Wesley Publishing Company.
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