Researchers wanted to determine if there was an association between daily coffee consumption and the occurrence of skin cancer. The researchers looked at 93,676 women enrolled in the Women’s Health Initiative Observational Study and asked them to report their coffee-drinking habits.
The researchers also determined which of the women had nonmelanoma skin cancer. After their analysis, the researchers concluded that consumption of six or more cups of caffeinated coffee per day was associated with a reduction in nonmelanoma skin cancer.
(a) What type of observational study was this? Explain.
(b) What is the response variable in the study? What is the explanatory variable?
(c) In their report, the researchers stated that “After adjusting for various demographic and lifestyle variables, daily consumption of six or more cups was associated with a 30%
reduced prevalence of nonmelanoma skin cancer.” Why was it important to adjust for these variables?