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The study has no clear indication of whether the researchers accurately measured the exposures and outcomes. Hence, it gives us no solid evidence to determine whether the researchers were biased or not biased in their data analysis. The study accounted for all confounding factors and they used the stratified research design to ensure all confounding factors are considered in the data analysis. It is hard to tell if the follow up was complete as the article does not address whether some respondents dropped out of the study. However, the follow up in general was long enough to ensure conclusive results.Results from the study show that tobacco smoking causes lung cancer. To be precise according to the study it decreases the life span of a person up to 10 years. The loss of year’s increases as one smokes for a long time, it is somehow time dependent. I do believe the results are valid and tobacco causes lung cancer. However, the quantification of the number of years it reduces is not appropriately measured as people usually have different immunities and smoking habits thus the effects would be different from one person to the