
than the worst trial of the other (see Figure A.1). In some cases there is
considerable overlap of scores in the conditions that are being compared.
The question is, then, whether the differences between the means are
large relative to the variance of the scores.
Descriptive Statistics
There are two kinds of statistics: descriptive and inferential. Descriptive
statistics give information about the particular sample of observations.
For instance, if 20 measures are taken, say, 20 runs of an algorithm where
fitness is recorded after 1,000 iterations, then a mean, usually symbol-
ized as
x
, can be calculated as the sum of the values divided by their
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