In a defined group of people (also referred to as a population), we happen to know that some percentage of the group has a disease (this is called the prior probability). For example, we might know from past studies that about 4% of women who are age 40 have breast cancer.
The circles below represent 100 people out of some defined population. You can indicate here that % people in that defined population have a disease. We can go through each of the 100 people below one by one and calculate the probability they have this disease (each time you adjust a value, whether or not a patient has a disease is recalculated).
Next: sensitivity of tests