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From a mathematical point of view, variance is a measure for the amount of spread one finds in observed data. When measuring any variable multiple times, the value for each observation may differ. The variation may be due to error, or to actual  actual variation in the measured object. The former type of variation is observed for instance when one asks a class of school children to indicate a length of one meter with their hands: one will find that each estimated length has some deviation with regard to an exact meter. The latter type of variation one finds for instance in a taking the height of individuals in a group of people.