Measuring the performance of assessors: Adjusting for the difficulty of the assessing task

Measures of assessor performance, such as the coefficient of intra-area dispersion, share a common deficiency. They fail to take into account the difficulty of the assessing task that often varies greatly from one district to another. The resulting standards for success are therefore rather arbitrary and inflexible. They also provide inadequate guidance for evaluating assessors and for targeting areas where improvements should be made. This paper presents a method to adjust for variations in the difficulty of the assessing task. This permits meaningful comparisons to be made concerning assessor performance across heterogeneous districts. The results can be applied both to policy analysis and internal management distinguishing between good assessments and good assessing. The method is general in nature, is applicable in different institutional settings and is not dependent on any particular measure of assessment uniformity. While the coefficient of dispersion (CD) is used in this presentation, alternative basic measures could be used with few modifications. The technique first is developed and then applied to the property tax system in Illinois. The example application suggests that several local assessors in districts with relatively high CDs are actually doing a rather good job when the difficulty of the assessment task is considered.