Int. J. Environ. Res. Public Health 2011, 8 2.4. Changes in Location of Last Purchase between Waves 5 and 6 1678 Changing purchase location (full tax vs. low/untaxed venue) among smokers in the baseline and first follow-up interview (waves 5 and 6) was used as a predictor cessation (at wave 7) following long term use of this price minimizing behavior. Differences in purchase location for current smokers at waves 5 and 6 were categorized as follows: (1) full tax source at both waves; (2) full tax source at wave 5, low/untaxed source at wave 6; (3) low/untaxed source at wave 5, full tax source at wave 6; and (4) low/untaxed source at both survey waves. Outcomes assessed include making quit attempts and cessation between waves 6 and 7. 2.5. Covariates Other covariates included in all multivariate analyses were country of residence (Canada, the United States, the United Kingdom, and Australia), age at time of interview (18–39, 40–54, 55+), sex (female, male), minority status (white, English speaking vs. minority defined as non-white, or non-English speaking in Australia), and Heaviness of Smoking Index (composite of cigarettes per day and time to first cigarette after waking; respondent assigned a number ranging from 0 to 6, with 6 relating to a heavier addiction). 2.6. SES Composite Variable In this study, a composite SES variable was created by combining each respondent‘s educational attainment and annual household income. Educational attainment varied by country due to different education systems. A derived measure of educational attainment was created which categorized respondents into low (completed high school or less), moderate (training, technical school, some university) or high education (university degree or higher) while taking into account varying educational systems between the four countries. Average annual household income was defined as the total income before taxes for all persons in the household combined. A derived measure of household income was categorized into three levels after taking into account different monetary measures and included: low income (≤$30,000/£15,000), moderate income ($30,000–59,000/£ 15,000–30,000), or high income (≥$60,000/£30,000). Those who did not provide their data on their educational status or annual income were coded as being non-responders and were not included in the analyses. The SES composite measure combined income and education into a low, moderate, high scale. Participants with low education and low income were categorized as having ―low‖ SES. Those with any combination of moderate or high education and income were deemed to have ―high‖ SES. All other combinations of income and education were categorized as being ―moderate‖ SES. For the main study population (n = 4,988), 21.0% had low SES, 53.6% had moderate SES, and 25.4% had high SES at baseline. SES could not be assessed in 5.9% of the main study population due to missing data in either reported income or education at baseline. Missing income data accounted for the majority of missing values in SES.