The whiskers are drawn outwards as far as observations called adjacent values. However, as you will see in the next step, some observations may be classified as potential outliers and in fact the whiskers extend only to cover observations which are not classified as potential outliers. Essentially, each whisker extends outwards from the edge of the box as far as the most extreme observation. These are lines drawn parallel to the scale (so they are horizontal in this course). The vertical line inside the box is located at the median. The ‘box’ is a rectangle with edges defined by the lower and upper quartiles so it indicates where the ‘middle 50%’ of the data can be found. The median of this data set is 110, and the lower and upper quartiles are 79 and 162, respectively. The median and quartiles are used to construct the ‘box’. Since the minimum is 66 and the maximum is 414, a scale from 0 to 500 (say) is suitable in this case.
The steps involved in constructing the boxplot in Figure 1.1 for the data set of β endorphin concentrations are as follows.įirst, a convenient scale is drawn covering the extent of the data. The easiest way to understand exactly what a boxplot represents and how it is constructed is to think about how you would draw one by hand. (1987) Beta-endorphin: a factor in 'fun run' collapse? British Medical Journal, 294, 1004.) (Data sourced from Dale, G., Fleetwood, J.A., Weddell, A., Ellis, R.D.