Table 3.

Subgroup analysis of the outcomes

Study group	Outcomes
	Number of studies (n)	Number of participants (n)	test of association			Test of heterogeneity
			RR	95%CI	p value	Model	p value	I2	I2 test for subgroup differences (%)	p for the interaction between subgroup and treatment
difficult intubation
Study design
Cohort	3	5567	3.41	0.88–13.25	0.08	RE	0.14	50	23	0.25
Case-control	2	95,407	1.50	1.03–2.18	0.03	RE	0.08	67
Position
Sniffing	2	394	5.77	2.29–14.58	0.0002	RE	0.63	0	89.8	0.002
Supine	3	100,570	1.30	1.17–1.44	< 0.00001	RE	0.59	0
difficult laryngoscopy
Study design
Cohort	6	9111	1.85	1.31–2.63	0.0005	RE	0.08	49	67	0.08
Case-control	3	103,277	1.34	1.22–1.48	< 0.00001	RE	0.87	0
Position
Sniffing	4	2210	2.0	0.97–4.15	0.06	RE	0.03	67	0	0.42
Supine	5	110,178	1.47	1.23–1.76	< 0.0001	RE	0.22	30
Mallamp-ati ≥ 3
Study design
Cohort	9	4189	1.9	1.12–3.21	0.02	RE	< 0.00001	85	0	0.6
Case-control	3	1489	1.62	1.27–2.08	0.0001	RE	0.61	0

CI confidence interval, RE random-effect model, RR risk ratio, I² a test for heterogeneity, I² > 50% indicates substantial heterogeneity