NHTSA crash investigation sampling system (CISS) is unreliable, inaccurate and does not estimate serious injury in motor vehicle crashes

Abstract

Objective

Errors and inaccuracies are documented in the 2017–2022 CISS (Crash Investigation Sampling System) field data managed by NHTSA on serious injury in motor vehicle crashes in the US.

Methods

All CISS cases of serious, non-fatal injury to 3–5 and 6–12 year old (yo) children were downloaded from the NHTSA website. Each case was summarized for the: 1) case identification by year, PSU and number, 2) case weighting factor and domain, 3) vehicle make, model and model year and 4) age, seating position and maximum injury severity (MAIS) to the child. Crashes with serious injury are in domains 02, 05 and 08 depending on the age of the vehicle. There were several cases with unusually high weighting factors (casewgt) in comparison to others in the groups. The sources of the high case weights were identified.

Results

There were 8 cases with MAIS 3–6 injury to 3–5 yo children. The weighted sum was 3,538.8 estimated national incidence for 6 years. Case #5 had a weight of 2,223.3, which was 63% of the sum. There were 8 cases with MAIS 4–6 injury to 6–12 yo children. The weighted sum was 5,817.9. Case #9 had a weight of 5,199.6, which was 89% of the sum. There were 18 cases with MAIS 3 injury to 6–12 yo children. Case #22 had a weight of 1,264.5 and case #25 937.8, representing 52% of the sum. The high case weights were traced to errors in the domain due to incorrect injury severity and vehicle age. NHTSA has been aware of the domain errors and unusually high case weights since 2012. They have not corrected the errors in high case weights. They have ignored the effects of errors in the CISS and NASS-CDS data on serious injury. The data errors lead to unrealistic and inaccurate estimates of serious injury. There are other problems with CISS and NASS-CDS.

Conclusions

There are pervasive errors in the CISS crash selection procedure, domains and case weights that make estimates of serious injury unreliable. There is no quality control to correct the data errors. CISS cases are incomplete and limited in comparison to the earlier NASS-CDS.

CISS sampling procedure is incorrect by relying on vehicle age for case selection. NASS-CDS sampling relied on occupant injury severity. CISS and NASS-CDS incorrectly handle multi-vehicle crashes. A complete review and revision of CISS and NASS-CDS is needed to correct errors so the databases are compatible from the 1980s to today. NHTSA needs to disclose information about the revised NASS-CDS and CISS files. The online NASS-CDS and CISS files are incorrect and inadequate to estimate serious injury in motor vehicle crashes.

CISS sampling frequencies rely on 2011 NASS-CDS, which is not relevant to serious injury crashes today. The 2011 NASS-CDS did not investigate older vehicles. CISS under-samples serious injury in older vehicles where race, gender and SES are important factors. CISS and NASS-CDS are not representative samples of serious injury in US crashes.