Calculate Coefficient of Dispersion (COD)

assesspy.cod(ratio)

COD is the average absolute percent deviation from the median ratio. It is a measure of horizontal equity in assessment. Horizontal equity means properties with a similar fair market value should be similarly assessed.

Lower COD indicates higher uniformity/horizontal equity in assessment. The IAAO sets uniformity standards that define generally accepted ranges for COD depending on property class. See IAAO Standard on Ratio Studies Section 9.1, Table 1.3 for a full list of standard COD ranges.

Note

The IAAO recommends trimming outlier ratios before calculating COD, as it is extremely sensitive to large outliers. The typical method used is dropping values beyond 3 * IQR (inner-quartile range). See IAAO Standard on Ratio Studies Appendix B.1.

Parameters:

ratio (numeric) – A numeric vector of ratios centered around 1, where the numerator of the ratio is the estimated fair market value and the denominator is the actual sale price.

Returns:

A numeric vector containing the COD of ratios.

Return type:

float

Example:

# Calculate COD:
import assesspy as ap

ap.cod(ap.ratios_sample().ratio)