disease probability

[bigcalidreams]

The PSA test is a simple blood test for prostate cancer in men. It is cheap,
but it is not very accurate. According to http://www.familymedicine.vcu.edu/resea ... index.html

Without having too look at the article, I know that 1 out of 90 men have cancer, but how do i figure out how many men out of 100 have cancer?
So if 100 men are considered:
(a) How many have cancer?
(b) Of those with cancer, what percentage test positive? What is the rate
of false negatives in the population of men with cancer?
(c) Of those without cancer, what percentage test negative. What is the
rate of false positives in the population of men without cancer?


I don't know where to start! Please guide me step by step.
Thanks a lot.

 

[Deleted member 4993]

One of the goal of this excersize is that you show your competence in gleaning the relevant data from an article.

So what is the relevant set of data?

 

[bigcalidreams]

Right, but I don't understand if 1 out of 90 men have cancer, what steps do i need to find have cancer out of 100?

 

[Deleted member 4993]

Right, but I don't understand if 1 out of 90 men have cancer, what steps do i need to find have cancer out of 100?

No

the article says -

with "negative" (normal) PSA test -- one out of 90 men have prostate cancer.

What does it say about men with "positive" (elevated) PSA test?

 

[JeffM]

Right, but I don't understand if 1 out of 90 men have cancer, what steps do i need to find have cancer out of 100?

As Suhobtosh Khan has told you, your ratio above is totally irrelevant to your question, but you do need to know how to change a ratio into a percentage to succeed in probability and statistics. Furthermore, that ratio may be relevant to some other question.

To turn 1/90 into a percentage, use the following method: (1/90) = (x/100), so 1 = 90x/100, so 100 = 90x, so x = 100/90 = (100 * 1)/90.
Generalizing, you can turn any ratio into a percent with the following formula: (y/z) = [100y/z] percent.
Of course, using that formula on (1/90) will NOT tell you what is the percentage of men who have had PSA tests and have prostate cancer because that ratio is irrelevant to THAT question in THIS case.