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5 | Page <br /> <br />studies enable us to track market trends in neighborhoods, popular housing types and classes of property. Within the county, we strive to achieve a ratio within the 93%-95% range for the median and mean. This allows us a margin to account for a fluctuating market and still maintain ratios within state mandated guidelines. Arithmetic Mean Ratio The mean is the average ratio. Unlike the median, the mean is influenced by outliers. We use this ratio not only to measure our assessment level, but also to analyze property values by municipality, type of dwelling and value range. These studies enable us to track market trends in neighborhoods, popular housing types and classes of property. Coefficient of Dispersion (COD) The COD is used to measure the accuracy of the assessment. The COD indicates the spread of the ratios from the mean or median ratio. The goal of a good assessment is a COD of 10 to <br />20. A COD under 10 is considered excellent and anything over 20 will result in an assessment review by the Department of Revenue. Price-Related Differential (PRD) The PRD is used to measure value related inequities in the assessment, referred to as regressively or progressivity. Appraised values are regressive if high value properties are under appraised relative to low value properties and progressive if high value properties are over appraised relative to the low value properties. The PRD is found by taking the mean (average) ratio of the sample and dividing by the weighted (aggregate) mean ratio. The acceptable range is .98 to 1.03.