what is the formula for the false positive and how do i calculate the predictive value positive?

Question

Neuroblastoma is a rare serious but treatable disease. The prevalence of this disease is 6.3%. Study of the screening test for neuroblastoma the VMA test a urine test gave the following data 

	Neuroblastoma- no	Neuroblastoma- yes
test results- negative	368	92
test results – positive	141	329

 Calculate the predictive value positive of the VMA test?

Answer 1

Predictive value positive is the probability that a person will actually have the disease given that they test positive on the screening test. In this case, it would be the probability that a person had a neuroblastoma and tested positive for a neuroblastoma.

Usually, a 2x2 table is set up where the disease outcome is along the top and the testing results are along the side. The only difference in your 2x2 is that the no/negative are in the place where people usually put the yes/positive, but as long as you understand the math behind PVP this is not a problem at all.

The formula is (number of people who truly had disease and tested positive)/(total number of people who tested positive for disease). Usually this is a/a+b, BUT since your 2x2 is set up a little differently the formula would be d/c+d or (329)/(329+141). Just an FYI - d/c+d is usually how people calculate predictive value negative, so always double check that you are choosing the cells that correspond to the correct people for the formula.

To figure out how many people were false positives, you would look at the intersection of neuroblastoma - no and test results - positive. In this case there were 141 false positives because they tested positive but did not actually have a neuroblastoma.

I hope this helped! Let me know if you have any additional questions.