# SOLVED - How to calculate the probability of disease given a positive test from only true positive rate, false positive rate, and rate of disease?

Here's the problem I'm trying to solve:

"Calculate the probability of having the disease for a person with a positive test result."

The information I have is:

True positive percent = 90%

False positive percent = 1%

Percent of disease in general population = 0.01%

I'm getting confused because I'm used to working with counts and I'm not sure I can use the same formulas when I only have ratios. Hints are welcome!

## 2 Answers

To help wrap your head around this very common problem, try starting with a sample of 1000 people. Make a 2x2 grid based on the reality of whether or not the person has the disease and what the test says. Use the probabilities given to fill out the grid with counts.

I'm sure your professor wouldn't want us to give more specific hints than that!

read about the Kaplan-Meier probability it will help you with formula part but it will require some tweaking.