A recent magazine article stated that bone mineral density (BMD) may one day predict breast cancer risk in older women, according to a study in the Journal of the National Cancer Institute. Researchers followed the health of 8,905 women who were 65 or older and initially cancer free. Eventually 315 participants developed breast cancer. Women with the highest BMD were almost three times more likely to develop breast cancer, compared to the women with low BMD. (Better Homes and Gardens, October 2001, page 224.) (1). Do the results of this study mean that a high bone mineral density (BMD) causes breast cancer in older women? Explain your answer. (2). Explain why it is not possible to conduct a randomized experiment in this situation.

1) Not neccesarily. There might be a small correlation, however, you cant predict if a person will have breast Cancer only due to their bone mineral density because the probability for a woman to have breast cancer is pretty low (below 5%). According to the data given, women with high bone density are just 3 times more likely to have breast cancer, thus, the probability will be just 15%. There might be a correlation but you need more information than just the mineral bone density to predict breast cancer, there might be other causes that are not related with mineral bone density.

2) It is impractical to conduct such an experiment. The problem lies in the fact that you cant fix the amount of mineral bone density a woman have. 2 women with different BMD might be quite different in different aspects one from the other, so you wont be completly sure that the cancer was caused in one case just because of the mineral bone density alone. An ideal experiment will have identical bodies differing on mineral bone density alone, but that is in practice impossible to do.