Published 3 September 2008, doi:10.1136/bmj.a196
Cite this as: BMJ 2008;337:a196

Endgames

Statistical question

Continuous variables

John Fletcher, clinical epidemiologist

1 BMJ, London WC1H 9JR

jfletcher{at}bmj.com

How would you best begin to explore the distribution of a continuous variable?

a) Scatter plot
b) Box plot
c) Histogram
d) Hypothesis test

Show Answers

Answer

c—A histogram displays the frequency of all the data divided into suitable intervals.

A histogram is perhaps the most commonly used graphical method of displaying either continuous data, such as height, or discreet data, such as mortality. The horizontal axis displays the limits that are used for each interval. For each interval a rectangular column centred on the midpoint is drawn rising from the horizontal axis. The vertical axis measures the number of values that fall within each interval or, if desired, the percentage of values that fall within each interval.

Scatter plots usually display the relation between two continuous variables plotted on the x and y axes. A one way scatter plot may be used to display a single continuous variable, but a disadvantage is that the plot may be difficult to read if too many points lie close together.

Box plots display only a summary of the data, and some of the information about the shape of the distribution is obscured. For example, a box plot may extend from the 25th centile to the 75th centile and contain a line representing the median. Lines projecting out from the box may be added ("whiskers") to display more extreme parts of the distribution of values.

Hypothesis testing is not a method for displaying data.

Cite this as: BMJ 2008;337:a196


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