Question 14
Draw and describe a box and whisker plot. (50% of marks) What is this used
for? (50% of marks)
College Answer
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
Discussion
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
References
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).