Data is usually presented as a table. Disease present or absent must be determined by some means and this is usually called the ‘gold’ or ‘reference’ standard. Positive Predictive Value answers the question about how likely a positive test is to represent disease.

Putting the numbers into our table gives a PPV of 48/60 = 80%

The NPV is 28/40 = 70%

This data is obtained from studies where the number of people with disease is known. In practice we don’t know who has disease or who hasn’t (otherwise we would not need to do a test!).