A blind study is one in which patients are randomly assigned into two groups, one of which receives the new drug or treatment being tested, while the other receives standard treatment. In some blind studies (though never in breast cancer studies), the new drug is compared to a placebo, which is an inactive pill that looks just like the drug being tested.
These clinical trials are called "blind" because patients don't know which group they're in. A double-blind study is one in which neither the participants nor the researchers know which group is receiving which treatment. Placebos aren't used when researching treatment for breast cancer or other serious diseases because they are never used in cases where not receiving treatment would endanger the participants in a trial.
