A tamoxifen study claims that a combination of chemotherapy and the hormone therapy tamoxifen is more effective than tamoxifen alone in preventing breast cancer. In the study, a group of researchers from the U.S. and Canada studied 1477 postmenopausal women with early-stage breast cancer. The women were randomly assigned to one of three study groups. Group A (361 patients) received tamoxifen alone on a daily basis for five years. Group B (566 patients) received chemotherapy and tamoxifen for six months, and then completed five years of tamoxifen treatment. Group C (550 patients) received chemotherapy alone for six months followed by five years of tamoxifen treatment. All 1477 patients knew which treatment they were receiving.
Which of the following is the most likely confounding variable in this study?
◦ The doctors running the study were only from the U.S. and Canada.
◦ Only women were in the study.
◦ Only 1477 women were involved in the study.
◦ The three groups in the study were of different sizes.
◦ Only postmenopausal women were in the study.
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