Box Plot


Health Economics

  • Sometimes a 'box and whisker' plot. This is a diagram in which the ends of the 'box' indicate the upper and lower values of the interquartile range (i.e. the middle 50 per cent) of a variable. The ends of the box are called 'hinges'. The distance between the hinges is called the 'H-spread'. The vertical line through the box indicates the median value. Values one 'step' outside the hinges are called 'inner fences', where a step is 1.5 times the difference between the hinges. Values two steps outside the hinges are called 'outer fences'. In the diagram, the minimum value lies inside the lower inner fence, which is not shown and the maximum value lies inside the upper outer fence, which is not shown. There are one or two 'adjacent values', which are observations just within the inner fences and these mark the extent of the 'whiskers' - the lines extending from either side of the box. Points beyond the extremities of the whiskers identify the maximum and minimum values (in some box plots, the ends of the whiskers are the two extreme values). In descriptive statistics the two extreme values, the two limits of the interquartile range and the median are sometimes referred to as the 'five-number summary' of the data. In some versions, the box plot is presented vertically.