[theamk]

no, that makes no sense. How does having identical genetics negate problem of different lifestyles?

Even the authors agree with awestley:

> Additionally, having a tattoo, especially among adolescents, has been suggested as an indicator of risky lifestyle highly associated with e.g. smoking [4] and alcohol consumption [46] – both risk factors of certain cancer types. Hence, evidence of an association between tattoo ink exposure and occurrence of cancer may be confounded by other health-related lifestyle factors. We intend to exploit the remainder of the information gathered in the survey in the future.

Indeed, it does seem like a big problem that should really be accounted in the initial analysis.