How can they know this?

[ElaineM]

Doctors can't know how long a person will live.
They have data from clinical trial results and other things that are purely related to science. They do not take human potential, a patient's attitude and what the patient is doing to help himself or her self into consideration.
For example if a person has is aggressive, has a positive attitude, eats healthy and exercises that will help a person live longer.
Doctors do not think about those things when making predictions.
A doctor gave me 6 months to live in January 2000, but I decided that was not going to happen.