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Su, Yuchen, Brian H. Stein, and John P. Robertson. 2023. “D-Ratios and Credibility in Experience Rating.” Variance 16 (1).
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  • Figure 1. 10th and 90th percentiles of mods by state
  • Figure 2. Quintile tests for high and low G-value NCCI gross experience rating ERA states
  • Figure 3. D-ratios by state
  • Figure 4. Primary and excess credibility by expected claim count


The American Academy of Actuaries defines credibility as a measure of the predictive value that the actuary attaches to a particular set of data in a given application. In currently common notation and terminology used for workers compensation experience rating, Zp and Ze denote primary and excess credibilities. This article shows for the first time that in NCCI’s Experience Rating Plan, the weight given to the reported primary loss ratio is ZpD, where D is the insured’s average D-ratio, and the weight given to the reported excess loss ratio is Ze(1D). We use this viewpoint to show an underlying similarity between NCCI’s and California’s workers compensation experience rating plans, to explain recent state-level performance of NCCI’s Experience Rating Plan, and to discuss interstate mods across states with differing experience rating plans.

© Copyright 2022 National Council on Compensation Insurance, Inc., John P. Robertson, and Yuchen Su. All Rights Reserved.

Accepted: November 21, 2021 EDT