TABLE 1.

Quality scale used to assess the included studies

Item assessed	Characteristic	Weight
Total sample size	100–1000	0
	1001‐10 000	1
	>10 000	2
Number of COPD never‐smoker cases	20–100	0
	101–300	1
	>300	2
Covariate adjustment (number)	2 (age and sex)	0
	>2	2
Study design	Cross‐sectional study	0
	Case–control study	1
	Cohort study	2
COPD diagnosis	Spirometry not registered or patient‐reported symptoms	0
	Pre‐bronchodilator FEV1/FVC < 70%	1
	Post‐bronchodilator FEV1/FVC < 70%	2
Total		10