Question 14

Draw and describe a box and whisker plot. (50% of marks) What is this used

for? (50% of marks)

Candidates either knew this topic or knew nothing about it. Some confused this with a Forrest

plot which scored no marks. This topic is well covered in many standard statistics books such as

Statistical methods for anaesthesia and intensive care / P S Myles, T Gin – 1

st Ed - Oxford:

Butterworth-Heinemann, 2001 ISBN: 9780750640657

The box and whisker plot is a way of graphically representing the "five number summary", or four parameters which demonstrate the central tendency of the data set. Correctly, the examiners point out that answering this question with a description of a forest plot would be disastrous.

The parameters:

**Range**(the minimum and maximum values)**First quartile**(25%)**Third quartile**(75%)**Median**, represented by the vertical bar in the centre

Uses of this graph:

- Good for comparing distributions (the centre, spread and overall range are immediately apparent)
- Useful for indicating whether a distribution is skewed
- Detects outliers

- Useful for summarising a set of data measured on an interval scale

Larsen, Russell D. "Box-and-whisker plots." *J. Chem. Educ* 62.4 (1985): 302.

Weisstein, Eric W. "Box-and-whisker plot." *From MathWorld–A Wolfram Web Resource.[Cited 2006 June 7]. Available from: URL: http://mathwold. wolfram. com/Box-and-Whiskerplot* (1999).