 Many types of statistical data exhibit a "variance-on-mean relationship", meaning that the variability is different for data values with different expected values.As an example, in comparing different populations in the world, the variance of income tends to increase with mean income.If we consider a number of small area units (e.g., counties in the United States) and obtain the mean and variance of incomes within each county, it is common that the counties with higher mean income also have higher variances.