In the last issue I talked about central tendency…what is often called the average. When we are looking at the results of a study that compares two or more conditions, the first thing we look at is the central tendency. Say we are looking at the difference in the ratings boys and girls give to a new zombie film. Boys give the film a median score of 8/10, while girls give it a median of 6/10.
This shows us that on average boys like the film more. But there is something else we really want to know as well when comparing the data for boys and girls. How closely are the boys’ and girls’ scores clustered around the central tendency? It might be, for example, that all the boys’ scores are quite close to the median, showing that they all had much the same reaction to the film. However, what if the girls’ scores ranged from 1 to 10? The fact that some girls really hated the film while others really loved it is actually just as interesting as the fact that the boys’ median rating was higher. This requires a measure of dispersion, which literally means how spread out or dispersed scores are.
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