An Unrecorded Sign Was Not Counted as an Absent Sign in a THC Case Review
In a retrospective THC-positive driver study, an unrecorded physiological sign was not counted as an absent sign. Percentages were calculated from reports that explicitly described the sign as present or absent, making the denominator central to interpretation.
Not every report contained every observation
Declues and colleagues reviewed physiological findings in selected THC-positive driver cases assessed by DRE and non-DRE officers. The series involved 363 cases, but each physiological variable was not documented in every report.
For an individual sign, the investigators used cases with an explicit presence-or-absence statement. It follows that a percentage for one sign may represent a different set of reports from a percentage for another sign.
Missing data are a third category
A blank entry cannot establish that an officer looked for a sign and found it absent. Conversely, it does not establish that the sign was present. Treating missing observations as negatives changes the question and can distort the estimated frequency.
Comparing sign percentages without their denominators can be equally misleading. A finding described often in the reports that mention it may still have been undocumented in many other cases. The missingness itself may also be related to examination or reporting practices.
Frequency does not establish specificity
The case series was selected around THC-positive investigations. A common observation within those cases does not show how rarely it occurs in drivers without THC or in people with other explanations for the sign.
When assessing a report, distinguish present, explicitly absent and not recorded. Then separate the observation from the explanation assigned to it. This preserves the study’s actual data structure without converting documentation gaps into physiological findings or individual impairment conclusions.
Related: How the reference specimen changes a cannabis accuracy claim.
Research cited
Declues K; Perez S; Figueroa A; Journal of Forensic Sciences; 2018. A Two-Year Study of Delta-9 Tetrahydrocannabinol Concentrations in Drivers; Part 2: Physiological Signs on Drug Recognition Expert and non-DRE Examinations.
