Application of Crash Modification Factors for Access Management, Volume 2: Research Overview (2021) / Chapter Skim
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Pages 182-188
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From page 182... ...
182 This appendix provides a summary of goodness-of-fit measures to assess the performance of SPFs, including the mean absolute deviation, modified R2, dispersion parameter, coefficient of variation (CV) of the calibration factor, and CURE.
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Overview of Goodness-of-Fit Measures 183 Figure C-2. Modified R2 value.
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184 Application of Crash Modification Factors for Access Management Variables in the equation shown in Figure C-5 are defined as follows: • CV = coefficient of variation of the calibration factor, • V(C) = variance of the calibration factor, and • C = estimate of the calibration factor.
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Overview of Goodness-of-Fit Measures 185 • Vertical changes. Large vertical changes in the CURE plot are potential indicators of outliers, which require further examination.
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186 Application of Crash Modification Factors for Access Management Hauer's book also states, "the overall fit of the SPF is best judged by the CURE plot for fitted values" (Hauer 2015, p.
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Overview of Goodness-of-Fit Measures 187 Assessment Tables While the CURE plot method works well for continuous variables, it is not applicable to variables with few categories (e.g., a database with speed limits of 45, 55, and 65 mph)
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188 Application of Crash Modification Factors for Access Management As a caution, if the constant calibration factor is not acceptable, but a calibration function shows less than 5 percent of CURE plot ordinates for fitted values exceeding the 2σ limits, this may be due to a small number of sites or crashes in the calibration dataset. Consider the sample size before adopting the calibration function.
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