The answer would be 0.99 chances of having the disease. But incase the disease is a very rare one, the answer maybe very surprising. Have a look! "Suppose that you are worried that you might

have a rare disease. You decide to get tested,

and suppose that the testing methods for this

disease are correct 99 percent of the time (in

other words, if you have the disease, it shows

that you do with 99 percent probability, and if

you don't have the disease, it shows that you do

not with 99 percent probability). Suppose this

disease is actually quite rare, occurring randomly

in the general population in only one of every

10,000 people.

If your test results come back positive, what are

your chances that you actually have the disease?

Do you think it is approximately: (a) .99, (b) .90,