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4.5 Reliability (Stability) of Cost-of-Injury Estimates

Cost Estimates Based on System-Provided Data Only

A cost estimate that is based exclusively on system-provided data is evaluated for statistical reliability (stability) by considering two basic sources of variation:

1. the count (or estimated count) of cases associated with the cost estimate;
2. the variation in assigned unit costs (medical and/or work loss) across the case records used for the cost estimate.

Under this approach, such estimates may be annotated as statistically unreliable (unstable) due to small contributing case counts, excessive case-to-case variation in assigned unit costs, or the combined effects of both.

For fatal injuries, any cost estimate based on 20 or fewer case records is considered statistically unreliable. Even when based on more than 20 case records, however, a cost estimate for fatal injuries can be statistically unreliable due to high variability of the case-level costs. The latter determination is based on the estimated coefficient of variation (CV) for the cost estimate, which measures its variability relative to its magnitude. If the estimated CV exceeds the threshold value of 0.22, the cost estimate is considered unreliable no matter how large the contributing case count.

For nonfatal injuries (i.e., hospitalized or ED treated-and-released), any cost estimate based on 20 or fewer surveillance records or an estimated national case count of 1200 or less is considered statistically unreliable. Even when based on a sufficiently large case count (surveillance / national estimate) a cost estimate for nonfatal injuries can be statistically unreliable due to high variability of the case-level costs. The latter determination is based on the estimated CV for the cost estimate. If the estimated CV exceeds the threshold value of 0.30, the cost estimate is considered unreliable regardless of the number of contributing surveillance records or the estimated national case count.

Note: In the present context, a lack of statistical reliability (stability) for a given cost estimate should be interpreted as meaning that the estimate would be likely to fluctuate substantially from one hypothetical base year (not index year) to the next. It does not mean that the estimate is biased or that it is based on inaccurate or unreliable data.

Cost Estimates That Combine System-Provided Data and User-Provided Data

Evaluating the statistical reliability (stability) of cost estimates requires substantial background knowledge about the data on which such estimates are based. While such evaluation is feasible for cost estimates that are based entirely on system-provided data, user-provided data will have differing origins and characteristics. The WISQARS Cost-of-Injury application therefore does not extend such evaluation to cost estimates that combine system-provided data and user-provided data.

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WISQARS Cost of Injury Reports