Table 2. Negative binomial regression model of the primary outcome.

Outcome	Median (interquartile range) per group	Range [Min, Max]	Percentage with any sickness respectively illness	Parameter	Maximum Likelhood Estimate (95% CI)	Exponential Estimate (95% CI)	P value
90 days sickness absence	30s Group: 0 (0–1)	[0, 62]	29,4%	Intercept	0.80 (0.49, 1.11)	2.23 (1.63, 3.03)	< .0001
	60s Group: 0 (0–1)	[0,29]	34,0%	Group (intervention groups vs control)*	-0.35 (-0.58, -0.12)	0.71 (0.56, 0.89)	0.003
	90s Group: 0 (0–1,5)	[0,40]	33,1%	Regular physical activity (yes vs no)*	-0.42 (-0.70, -0.15)	0.65 (0.5, 0.86)	0.003
	Control Group: 0 (0–2)	[0,51]	34,8%	Dispersion	4.64 (4.17, 5.15)
90 days illness	30s Group: 2 (0–7)	[0,56]	65,0%	Intercept	1.27 (1.14, 1.39)	3.55 (3.13, 4.02)	< .0001
	60s Group: 2 (0–6)	[0,60]	63,3%	Group (intervention groups vs control)*	-0.12 (-0.26, 0.01)	0.89 (0.77, 1.01)	0.073
	90s Group: 2 (0–6)	[0,70]	64,5%	Gender (Male vs Female)*	-0.15 (-0.27, -0.04)	0.86 (0.76, 0.96)	0.0097
	Control Group: 2 (0–7)	[0,90]	69,3%	Dispersion	1.53 (1.41, 1.66)

This study investigated the effect of cold showering on health and work: a trial randomizing a (hot-to-) cold shower for 30, 60, 90 seconds or a control group during 30 consecutive days followed by 60 days of showering cold at their own discretion for the intervention groups.

*The exponential of the estimates are Incident Rate Ratios (IRR)