Independent variable: the one that you change.

Dependent variable: the one that you measure.

For example, a scientist wants to find out whether a diet including oats makes a horse run faster, so she adds oats to one horse's diet for a week before the race. The other horse receives the same diet as usual. Both horses race on the planned day and she records the time it takes for both of them to run 1km.

In this case, the independant variable is the diet of the horse - this is what the scientist changed in the experiment. The dependent variable is the time it took for each horse to run - this is what the scientist measured.

Understanding variables is a key part of sound experimental design, without which we cannot rely on the results we produce.

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