Table 2.

Subgroup analysis of effect of smoking on the thickness of retina and choroid.

Outcome of interest	Subgroups	No. of studies	Summary effect		Heterogeneity
			SMD (95% CI)	P value	I 2, %	P value
Retina	Central retina
	Region
	Europe	8	−0.019 (−0.193 to 0.156)	0.833	0	0.636
	Africa	1	−0.528 (−0.810 to −0.246)	<0.001	—	—
	Study design
	Case-control	5	−0.246 (−0.606 to 0.114)	0.180	66.8	0.029
	Cross-sectional	4	0.016 (−0.214 to 0.247)	0.889	0	0.919
	Detection
	Heidelberg	3	0.074 (−0.146 to 0.294)	0.511	0	0.869
	Cirrus	2	−0.242 (−0.606 to 0.121)	0.192	29.6	0.233
	Others	4	−0.334 (−0.782 to 0.114)	0.249	47.6	0.064
	Retinal nerve fiber layer
	Average	3	−0.332 (−0.637 to −0.027)	0.033	58.8	0.063
	Inferior	2	−0.632 (−1.092 to −0.172)	0.007	66.9	0.082
	Superior	2	−0.720 (−0.977 to −0.463)	<0.001	0	0.886
	Nasal	2	−0.406 (−1.161 to 0.349)	0.292	87.5	0.005
	Temporal	2	−1.621 (−4.402 to 1.161)	0.253	98.5	<0.001
	Ganglion cell layer
	Average	2	−0.229 (−0.470 to 0.012)	0.062	15.4	0.307
	Superotemporal	1	−0.341 (−0.762 to 0.080)	0.112	—	—
	Superior	2	−0.549 (−0.884 to −0.215)	0.001	51.8	0.15
	Superonasal	1	−0.222 (−0.641 to 0.198)	0.300	—	—
	Inferotemporal	1	−0.104 (−0.522 to 0.314)	0.625	−	−
	Inferior	2	−0.602 (−0.938 to −0.265)	< 0.001	61	0.109
	Inferonasal	1	−0.228 (−0.647 to 0.191)	0.286	—	—
Choroid	Region
	America	1	−0.499 (−0.829 to −0.170)	0.003	—	—
	Europe	5	−0.195 (−0.431 to 0.041)	0.105	85.2	<0.001
	Africa	1	−0.291 (−0.570 to −0.012)	0.041	—	—
	Study design
	Case-control	3	−0.329 (−1.517 to 0.859)	0.587	92.5	<0.001
	Cross-sectional	3	−0.228 (−0.450 to −0.006)	0.044	0	0.762
	Detection
	Heidelberg	2	−0.716 (−1.435 to 0.003)	0.051	86.3	0.001
	Cirrus	1	−0.096 (−0.675 to 0.483)	0.746	—	—
	Others	4	0.027 (−0.399 to 0.454)	0.9	65.5	0.055

SMD: standard mean difference; CI: confidence interval. The outcome marked as bold demonstrated significant effect.