Often one needs to know about the extent of variability of the individual measurements from the mean. A comparison of the average yearly temperature between Albany and San Francisco may not tell the complete story. Albany temperature fluctuates a lot more than that of San Francisco, even though the average temperature may be very similar. Defined as the difference between third and the first quartile. Any other distance measures based on percentiles, e.g., difference between 90 th .
Variance is the arithmetic mean of the squares of deviations of the observations from their mean.
Variance is a measure of dispersion. It has the same unit as that of the measurements.