Int. J. Environ. Res. Public Health 2011, 8 1688 education data. Therefore, it is possible that the composite measure is not completely representative of each participant‘s true SES. More detailed SES measures may be needed to fully understand the relationship between price minimizing behaviors and cessation across different stratum of SES. Several policy interventions can be implemented that specifically address the availability of cheaper tobacco products. Examples include those policies which are called for or based on aims set forth in the Framework Convention on Tobacco Control (FCTC), such as those to initiate large scale efforts to eliminate illicit trade of tobacco or those banning duty-free sales. Both of these policy interventions aim to reduce the supply of lower priced tobacco products [30]. In addition, the tobacco industry often targets specific population subgroups with price promotion materials, which also serve to stimulate in-store sales [5,31]. Comprehensive marketing bans, including bans on price promotions, can help to reduce the availability of cheaper tobacco products. Setting minimum price laws which prohibit promotional incentives such as buy-downs or other programs may also effectively increase cigarette prices at the retail level, while also reducing use of promotional materials [31]. Recently, the United States Government passed the PACT Act which prohibits common courier services, including the United States Postal Service (USPS) from distributing cigarettes and smokeless tobacco products which may not have proper taxes assessed [32]. However, other lower priced venues still provide cheaper alternatives which must be addressed through additional policy interventions. 5. Limitations Although this study has several strengths including a robust sample size of international participants, detailed purchasing behavior information, and longitudinal data that has been validated for use in this international population, there are also limitations. Firstly, the ITC study relies on self reported data. However, this study has been pilot tested and validated for use in all four countries using self-reported responses. Another limitation of the ITC study is the attrition rates between survey waves. Among all smokers interviewed at baseline, approximately 29% were lost to follow-up one year later. After examining baseline characteristics between lost and retained respondents, a greater proportion of those lost to follow-up were younger, from minority racial or ethnic groups, and had slightly lower income and education. Although differences existed, the magnitude of difference was not substantial, and all analyses were weighted to be representative of the population within each country [24]. Therefore, it is unlikely that attrition has influenced the results presented here. Supplementary analyses found that tobacco prices did not confound the relationship between use of price minimizing behaviors and cessation. Although self-reported and exogenous measures of tobacco price were significantly correlated, neither measures of tobacco price may be fully sufficient to examining this relationship. Further research is needed to develop a more representative price measure that can be used to elucidate this relationship. Analyses in this manuscript also relied on a composite SES measure that was developed using self-reported categorical data on income and education. Therefore, it is possible that the composite measure is not truly representative of each participant‘s SES status. In addition, respondents with missing data in the SES composite measure were not included in the analysis and could introduce bias. However, missing data were evenly distributed between persons who did and did not engage in price minimizing behaviors. Therefore, any bias introduced would most likely be non-differential, and point