Calculate the county's Price Related Differential (PRD), which is a measure of the progressivity of the property tax. The PRD is calculated by dividing the mean assessment ratio (calculated in #1 above) by the aggregate assessment ratio, which is calculated by dividing the sum of all assessed values by the sum of all market values. A PRD of 1.0 indicates no assessment bias according to value. A PRD greater than 1.0 suggests that higher value properties are under assessed relative to lower.