For what percent of those tested for a certain infection was the test accurate; that is , positive for those who had the infection and negative for those who did not have the infection?
1) of those who tested positive for the infection, 1/8 did not have the infection.
2) of those tested for the infection, 90 percent tested negative.
Agree, and I think this problem is best solved with a Double-Matrix
Just use Positive/Negative and Tested Positive/Tested Negative for each axis.
Then you will see that we are actually lacking the ones who tested negative.
Try it and let me know
Hope it helps