Treating breast cancer Early on, the most common treatment for breast cancer was removal of the breast.

It is now usual to remove only the tumor and nearby lymph nodes, followed by radiation. The change in policy was due to a large medical experiment that compared the two treatments. Some breast cancer patients, chosen at random, were given one or the other treatment.

The patients were closely followed to see how long they lived following surgery. What are the explanatory and response variables? Are they categorical or quantitative?