This is a common query in statistics. When you have a situation such as several individuals carrying out a task, like moving through a maze to a reward - a task which is timed before the reward is introduced and again afterwards - the interest in the experiment is whether the food reward has an effect on the mice's speed of running the maze, or solving the problem that the maze presents them with. The method used for calculating the effect, if any, is a paired t-test.
