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Figure 5.4. The mean performance of three age groups on a test of mental
arithmetic with silence or loud rock music in the background; vertical lines show
the standard errors of the means.
This is an example of an interaction graph: it shows the way two variables, age
and background noise, interact with each other.
Error bars
Figures 5.3 and 5.4 both have vertical lines called error bars giving a measure of
the amount of variation in the scores of individual participants around the group
mean. Conventionally, error bars either show either the standard error, as here, or
the 95% confidence interval (see Appendix 5).
Exercise 5.1 Deciding on the best way to show data
Figures 5.3 and 5.4 show exactly the same data in two different ways.
• Assuming the data are real (which they are not) what would you conclude
about the effect of loud music on the ability to do mental arithmetic?
• Now decide which figure shows this conclusion most clearly.
See Appendix 2 for answer and comment.
RESEARCH PROJECTS AND DISSERTATIONS 151