Table 3

Linear regression-derived coefficients (B) and 95% CIs, with overall quality score as the dependent variable for included abstracts

Predictor	Category/unit	Univariable			Multivariable
Predictor	Category/unit	B	95% CI	P value	B	95% CI	Tolerance	VIF	P value
Year	1 year	0.07	(−0.22 to 0.36)	0.632
No of authors	1 person	0.10	(0.02 to 0.18)	0.019	0.05	(−0.03 to 0.13)	0.87	1.15	0.216
Continent	North America	Reference
	Europe	0.74	(−0.08 to1.56)	0.078
	Oceania	0.54	(−0.28 to 1.36)	0.197
	Asia	0.73	(−0.37 to 1.83)	0.191
	South America	−0.39	(−2.16 to 1.37)	0.661
Type of Intervention	Pharmacological	Reference			Reference
	Non-pharmacological	−1.02	(−1.53 to -0.51)	<0.001	−0.70	(−1.27 to to -0.12)	0.73	1.37	0.018
No of centres	Single centre	Reference
	Multicentre	0.45	(−0.35 to 1.25)	0.267
Word count	1 word	0.00	(−0.01 to 0.01)	0.923
Research area	Insomnia	Reference			Reference
	Sleep-related breathing disorders	0.78	(0.25 to 1.31)	0.004	0.42	(−0.13 to 0.97)	0.77	1.30	0.135
	Others	0.42	(−0.19 to 1.03)	0.179	−0.17	(−0.80 to 0.47)	0.77	1.30	0.599
Sample size	1 participant	−0.001	(−0.00 to 0.00)	0.139
Funded by industry	Yes	Reference			Reference
	No	−1.08	(−1.58 to -0.58)	<0.001	−0.66	(−1.24 to -0.08)	0.72	1.40	0.025

For multivariable analysis, constant=6.106, R²=0.153, adjusted R²=0.128, p<0.001.
VIF, variance inflation factor.
Bold values are those indicating statistical significance.