Researchers wanted to determine whether there is an association between mobile phone use and body mass index. To do so, 105,028 men and women aged 18 years or over from the United Kingdom were recruited and their cell phone use behavior was studied (number of calls per day, number of hours per week, year cell phone was first used) along with other variables (amount of exercise, body mass index) of the individuals. The researchers found a strong positive association between duration of phone calls on a cell phone and body mass index (that is, as the duration of phone calls increases, body mass index tends to increase as well).

(a) What type of observational study is this?

(b) Many studies involving cell phones look for a link between cell phone usage and negative health outcomes (such as stroke or cancer) due to radio-frequency exposure. The following quote is from the article: “Obesity is associated with health outcomes such as stroke and cancers, which are of interest in relation to radio frequency exposure, and therefore is potential for confounding.” Explain what this means.