Breast-cancer screening A study published in 2010 in the New England Journal of Medicine discusses a breast-cancer screening program that began in Norway in 1996 and was expanded geographically through 2005. Women in the study were offered mammography screening every two years. The goal of the study was to compare incidence-based rates of death from breast cancer across four groups:

1. Women who from 1996 through 2005 were living in countries with screening.

2. Women who from 1996 through 2005 were living in countries without screening.

3. A historical-comparison group who lived in screening countries from 1986 through 1995.

4. A historical-comparison group who lived in nonscreening countries from 1986 through 1995.

Data were analyzed for 40,075 women. Rates of death were reduced in the screening group as compared to the historical screening group and in the nonscreening group as compared to the historical nonscreening group.

a. Is this an observational or experimental study?

b. Identify the explanatory and response variable(s).

c. Does the study prove that being offered mammography screening causes a reduction in death rates associated with breast cancer? Why or why not?