Table 4.
Optimal Thresholds for Predicting Adverse Outcomes and Associated Odds Ratios for Driving Outcomes
| Item | Description | Threshold | Sens. | Spec. | OR [95% CI]β | P |
|---|---|---|---|---|---|---|
| Severe driving event: emergency brake | ||||||
| KSS | Subjective sleepiness | 7 | 1.0 | 0.75 | 20.21‡ [2.4, 168.1]β | 0.001 |
| LFA | Likelihood of falling asleep | 3 | 0.54 | 0.75 | 20.46‡ [2.5, 170.3]β | 0.001 |
| SSQ1 | Struggle to keep eyes open | 6 | 0.83 | 0.90 | 46.00‡ [5.1, 413.5] | <0.001 |
| SSQ2 | Vision blurred | 3 | 1.0 | 0.77 | 22.18‡ [2.7,184.7]β | <0.001 |
| SSQ3 | Nodding off to sleep | 2 | 1.0 | 0.64 | 12.23‡ [1.5, 101.3]β | 0.006 |
| SSQ4 | Difficulty keeping to middle of road | 5 | 0.83 | 0.90 | 43.33‡ [4.8, 388.8] | <0.001 |
| SSQ5 | Difficulty maintaining correct speed | 4 | 0.83 | 0.84 | 25.91‡ [2.9, 229.0] | 0.003 |
| SSQ6 | Mind wandering | 6 | 0.83 | 0.89 | 41.36‡ [4.6, 370.4] | <0.001 |
| SSQ7 | Reactions are slow | 4 | 0.83 | 0.86 | 29.00‡ [3.3, 257.0] | 0.002 |
| SSQ8 | Head dropping | 3 | 0.5 | 0.88 | 7.16 [1.4, 37.4] | 0.02 |
| SSQ global score | 34 | 0.83 | 0.90 | 41.59‡ [4.6, 372.4] | <0.001 | |
| Moderate driving event: lane deviation | ||||||
| KSS | Subjective sleepiness | 5 | 0.54 | 0.69 | 2.84 [1.6, 5.1] | 0.001 |
| LFA | Likelihood of falling asleep | 3 | 0.38 | 0.78 | 2.08 [1.1, 3.9] | 0.022 |
| SSQ1 | Struggle to keep eyes open | 2 | 0.66 | 0.49 | 1.99 [1.1, 3.6] | 0.025 |
| SSQ2 | Vision blurred | 2 | 0.54 | 0.62 | 1.95 [1.1, 3.5] | 0.024 |
| SSQ3 | Nodding off to sleep | 2 | 0.50 | 0.68 | 2.11 [1.2, 3.80] | 0.013 |
| SSQ4 | Difficulty keeping to middle of road | 2 | 0.75 | 0.44 | 2.23 [1.2, 4.2] | 0.013 |
| SSQ5 | Difficulty maintaining correct speed | 2 | 0.63 | 0.47 | n.s | n.s |
| SSQ6 | Mind wandering | 3 | 0.49 | 0.61 | n.s | n.s |
| SSQ7 | Reactions are slow | 2 | 0.60 | 0.49 | n.s | n.s |
| SSQ8 | Head dropping | 2 | 0.35 | 0.79 | 2.13 [1.1, 4.0] | 0.020 |
| SSQ global score | 12 | 0.75 | 0.5 | 3.12 [1.6, 5.9] | <0.001 | |
Dichotomous OR—change in odds of a severe/moderate event occurring in the next 15 minutes when the predictor value is above the identified threshold, compared to when below.
βOdds ratio calculated with Haldane-Anscombe Correction and significance via Fishers Exact Test.
‡Odds ratio is inflated due to low n in the false negatives section of the contingency table. Interpretation/reporting of the lower bound 95% confidence interval is recommended.