Int. J. Environ. Res. Public Health 2011, 8 1675 depending on the primary language of the participant. Strict protocols were followed to ensure methodological congruity across research facilities and between the two languages [25,26]. A strength of the ITC-4 project is the longitudinal study design. However, this design is also subject to attrition over time. To compensate for respondents who are lost to follow up at each survey wave, lost cohort members are replenished by newly recruited respondents following similar recruitment strategies as the original cohort. Replenishment is done to ensure at least 2,000 participants from each country are present at each survey wave. Data are weighted to adjust for the changing demographic characteristics over time due to the replenishments of the cohort [26]. 2.2. Study Populations A total of 7,038 smokers were interviewed as part of the wave 5 data collection in ITC. Study participants included in this analysis are smokers at baseline who were re-contacted one year later at the wave 6 interview (n = 4,988). Wave 5 data were collected from October 2006 until February, 2007. Wave 6 data collection began in September 2007 and was completed in February 2008. Among the participants included in this study population, 1,245 (25.0%) were in Canada, 1,130 (22.7%) were in the US, 1,263 (25.3%) were in the UK, and 1,350 (27.1%) were in Australia. In the current study, baseline demographic characteristics between smokers who were retained in the cohort and those who were lost to follow-up at wave 6 were generally similar, suggesting random loss to follow-up among this study population. The demographic and behavioral characteristics of this study population are described in Table 1. Table 1. Demographic Characteristics of participants who were current smokers present in waves 5 (main/recruitment) and at wave 6 follow-up, overall (n=4988) and stratified by country (CA: n = 1245; US: n = 1130; UK: n = 1263; AU: n = 1350) (n, % [unweighted]). Overall (n = 4,988) CA (n = 1,245) US (n = 1,130) UK (n = 1,263) AU (n = 1,350) 1,245 (25.0) 1,130 (22.7) 1,263 (25.3) 1,350 (27.1) 2,867 (57.5) 2,121 (42.5) 1,272 (25.5) 2,069 (41.5) 1,647 (33.0) 4,509 (90.5) 472 (9.5) 723 (58.1) 522 (41.9) 318 (25.5) 542 (43.5) 385 (30.9) 1,144 (91.9) 101 (8.1) 680 (60.2) 450 (53.9) 208 (18.4) 487 (43.1) 435 (38.5) 955 (84.8) 171 (15.2) 715 (56.6) 548 (43.4) 315 (24.9) 482 (38.2) 466 (36.9) 1,203 (95.5) 57 (4.5) 749 (55.5) 601 (44.5) 431 (31.9) 558 (41.3) 361 (26.7) 1,207 (89.4) 143 (10.6) Characteristic Country Canada United States United Kingdom Australia Sex Female Male Age at Wave 5 18–39 40–54 55+ Ethnicity White Non-White