Table 2 Mean (SD), paired t values, and effect size of the NFR and CIS at baseline and two year follow up among the drivers with (natural intervention group) and without (comparison group) an increase in working hours during the follow up.
| Baseline (before increase in working hours) | Two year follow up (after increase in working hours) | Difference score (follow up–baseline) | Paired t value | Effect size | n | |||
|---|---|---|---|---|---|---|---|---|
| Natural intervention group (increase in working hours) | ||||||||
| NFR, mean (SD) | 38.1 | (31.3) | 50.7 | (37.0) | 12.6 (30.6) | 4.20* | 0.40 | 104 |
| CIS, mean (SD) | 52.3 | (23.7) | 62.1 | (26.3) | 9.8 (24.9) | 3.99* | 0.41 | 103 |
| Comparison group (no increase in working hours) | ||||||||
| NFR, mean (SD) | 31.2 | (31.1) | 32.7 | (32.9) | 1.5 (26.5) | 1.12 | 0.05 | 393 |
| CIS, mean (SD) | 46.5 | (20.3) | 50.1 | (21.5) | 3.6 (19.3) | 3.68* | 0.18 | 381 |
*p<0.05 (two‐tailed).
Effect sizes:32 large effect (0.81–1.00); moderate effect (0.51–0.80); small effect (0.20–0.50); clinically irrelevant effect (0.00–0.19).