Table 1. Description of the individual studies included in the meta-analysis.

Id	1st Author	Year	Country	Period of study	Population	Age (Years)	Sample size	Awareness of e-cig	Previous use of e-cig	Current use of e-cig	Healthier perception of e-cig	STROBE score
1	Adkison[19]	2013	U.S.,UK,CA,AU*	2010.7–2011.6	Current and former smokers	>18	5939	0.466	0.076	0.029	0.798(current smokers)	18
2	Choi[25]	2013	U.S.(MN,SD,ND,MI,KS)	2010.10–2011.3	Smokers and non-smokers	20–28	2624	0.699	0.07	0.012	0.53(awareness)	20
					smokers		532	0.85	0.285		0.593
					Former smokers		320	0.745	0.097		0.525
					Never smokers		1742	0.655	0.027		0.519
3	Dawking[20]	2013	33 countries (72% in Europe)	2011.9–2012.5	Males,END users	43 (M)#	1347				0.06	20
4	Dockrell[26]	2013	Britain	2012.2	Smokers	>18	2093	0.79	0.216	0.03		21
					Former smokers		4473	0.38	<0.01	<0.01
					Never smokers		5866	0.47	0.04	0.01
				2010.4	Smokers		1380		0.35		0.39
5	Etter[18]	2011	European union and U.S., CA	2010.3–10	Smokers and non-smokers, END users	41 (M)	3587		0.85	0.808	0.84(users)	20
					Smokers		1051		0.705	0.617	0.811(n = 740)
					Former smokers		2508		0.91	0.892	0.843(n = 2279)
6	Goniewicz[27]	2012	Poland	2010.9–2011.6	Smokers and non-smokers	15–24	13787	0.864	0.209	0.069	0.548	21
					Smokers		4738	0.431	0.437	0.153
					Non-smokers		9022	0.087	0.087	0.024
					Smokers and non-smokers	20–24	1894		0.19	0.059
7	King[28]	2013	U.S.	2011.7–8	Smokers and non-smokers	>18	4050	0.579(0.558–0.600)	0.062(0.052–0.073)			18
					Smokers			0.769(0.722–0.815)	0.212(0.170–0.254)
					Former smokers			0.654(0.617–0.691)	0.074(0.050–0.097))
					Never smokers			0.501(0.473–0.529)	0.013(0.007–0.018)
8	Kralikova[29]	2012	Czech	2011.10	Smokers	32 (M)	973	0.86	0.26	0.07		8(Letter)
9	Li[30]	2013	New zealand	2011.3–5	Current and former smokers	>18	480	0.07			0.33	20
10	McMillen[31]	2012	U.S.	2010.9–11	Smokers and non-smokers	>18	3240		0.018	0.003546		21
					Smokers		580		0.067241379
					Former smokers		787		0.015247776
					Never smokers		1802		0.003329634
11	Pearson[32]	2012	U.S.	2010.6	Smokers and non-smokers	>18	2649	0.402	0.03	0.01		21
					Smokers		1308	0.571	0.114(0.093–0.140)	0.041(0.03–0.056)	0.432
					Former smokers		661	0.415	0.02(0.010–0.038)	0.0049 (0.0013–0.018)	0.46
					Never smokers		680	0.325	0.77(0.0035–0.017)	0.0029 (0.001–0.008)
				2010.1–4	Current and ever smokers	18–49	3658	0.58	0.06
					Smokers		419	0.582	0.064(0.053–0.077)		0.46
					Formersmokers		3239	0.58	0.031(0.013–0.071)		0.449
12	Popova[33]	2013	U.S.	2011.11	Current and former smokers	42 (M)	1836		0.201	0.08		20
13	Regan[34]	2013	U.S.	2010.4–5	Smokers and non-smokers	>18	10328	0.322	0.027	0.01		19
					Smokers			0.496(0.459–0.533)	0.182(0.138–0.227)	0.063(0.041–0.086)
					Former smokers			0.307(0.281–0.334)	0.062(0.040–0.083)	0.029(0.014–0.045)
					Never smokers			0.283(0.262–0.303)	0.038(0.027–0.049)	0.022(0.013–0.031)
14	Richaedson[35]	2012	U.S.	2010.6	Current and former smokers	>18	1310		0.1		0.35	18
15	Sutfin[36]	2013	U.S.(NC)	2009.fall	Smokers and non-smokers	21 (M)	4444		0.049	0.02	0.45(users)	20
					Smokers		867		0.106
					Former smokers		882		0.074
					Never smokers		2253		0.012
16	Trummbo[37]	2013	U.S.(CO)	2011.4	Smokers and non-smokers	19–22	244	0.71	0.13			19
17	Vickerman[22]	2013	U.S.(CT,LA,NE,NC,SC,TX)	2012.1–10	Smokers	49 (M)	2476		0.309	0.09	0.045(users)	20
18	Pokhrel[38]	2014	U.S.(Hawaii)	?	Smokers	45.8 (M)	834		0.134			19
19	Tan[39]	2014	U.S.	2013.10–12	Smokers and non-smokers	49.5 (M)	1449		0.136			21
					Smokers		219		0.472
					Former smokers		422		0.127
					Never smokers		809		0.038
20	Zhu[40]	2013	U.S.	2012.2–3	Smokers and non-smokers	>18	10041	0.754	0.081	0.014	0.499(users)	19
					Smokers		3111		0.322	0.063
					Smokers			0.881(0.859–0.903)
					Never smokers			0.692(0.670–0.714)
21	Giovenco[41]	2014	U.S.	2013.6	Current and former smokers	>18	2136		0.468	0.161		18
					Former smokers				0.383(0.257–0.510)	0.139(0.047–0.231)
22	Christensen[42]	2014	U.S.(Kansas)	2012–2013	Current and former smokers	>18	9656		0.118	0.034		20
					Smokers		1341		0.45
					Former smokers		2593		0.105
					Never smokers		5690		0.022
23	Tan[43]	2014	U.S.	2012.10–2013.2	Smokers and non-smokers	>18	3630	0.771(0.745–0.797)n = 3487			0.507(0.478–0.537)(awareness, n = 2609)	21
					Smokers		586	0.886(0.844–0.928)			0.65(0.576–0.724)
					Formersmokers		939	0.783(0.743–0.823)			0.495(0.436–0.553)
					Never smokers		2052	0.730(0.686–0.773)			0.459(0.421–0.496)
24	Gallus[44]	2014	Italy	2013	Smokers and non-smokers	>15	3000	0.911	0.068	0.012		20
					smokers				0.204	0.037
					Former smokers				0.07
					Never smokers			0.89	0.026
25	Czoli[45]	2014	Canada	2012	Smokers and non-smokers	16–30	1188	0.434	0.161	0.057		20
26	Brown[46]	2014	Britain	2012	current and former smokers		4117	0.93	0.366	0.215	0.670	20
					Smokers		3538	0.934	0.365	0.219	0.676
					Former smokers		579	0.929	0.373	0.188	0.632
27	Vardavas[47]	2014	European union	2012	Smokers	>15	7352		0.203			20
					Former smokers	>15	5782		0.047
					Never smokers	>15	13432		0.012
28	Jose[48]	2014	Spain	2013–2014	Smokers and non-smokers	>16	736		0.065	0.016		21
					smokers		171		0.211	0.053
					Former smokers		267		0.041	0.007
					Never smokers		298		0.003	0.003

Note

*U.S.,UK,CA,AU: the United states, the united kingdom,Canada and Australia; #(M):mean age.