Excessive cumulative exposure to ovarian hormones is believed to cause breast cancer. A study (New England J. Medic. 2003; 348: 2313–2322) used information from 1811 pairs of female twins, one or both of whom had breast cancer. Paired twins were compared with respect to age at puberty, when breast cancer was first diagnosed, and other factors. Their survey did not show an association between hereditary breast cancer and hormone exposure.
a. What type of observational study was used by the researchers?
b. Why do you think the researchers used this design instead of a randomized experiment?