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3.2 Average and variability for a single non-parametric dataset

Data that are scale or ordinal, but which do not satisfy the criteria for parametric data, will not be described well by the mean as an average, and a symmetrical measure of variability such as confidence interval or standard deviation. A box plot (Fig. 6) is more complicated that an error plot, and displays the median value against an appropriate vertical scale, and the interquartile range as a vertical bar (the 'box'). Typically, the overall data range is also represented by a vertical line (sometimes referred to as a 'box and whisker' plot), although extreme values and outliers may be separated out from the overall data range.

Box plot

Figure 6. The main features of a box plot, including outliers or extreme values excluded from the range