How do doctors know how long someone with cancer might live?

We don't.: This is often a very difficult, if not impossible, question to answer for patients. Knowing someone's cancer type and stage and overall health, we can look at statistics of comparable patients and provide estimates of response to therapy, but this is, at best, an educated guess. Statistics are for populations, not for individuals--what does someone look like who has "70% survived" a cancer?

Many ways.: First, the patient's overall health can give an idea of how long he/she might live. In cases of cancer, tests are also done to see how malignant the cancer is, and how far it has spread. Statistcs have also been compiled to show how long patients with similar cancers at similar stages have lived, which can provide doctors with many tools for an accurate calculation.

Usually we do not: It is very hard to predict how long an individual with cancer may live. There are averages for different cancers, ages, genders etc, but there is wide variability from those averages making the prediction for an individual patient very difficult.

Estimation only: Doctors can make estimations for how long an individual lives based upon what the average person with a similar disease lives. Since none of us are perfectly average, there can be a wide variation and high inaccuracy of these estimations. Nevertheless, patients often want, and deserve to know a doctor's best estimation to assist them with planning. Always take this estimation with a grain of salt.