The question is confusing because a TRUE POSITIVE test result is finding detecting cancer by US (goes to show you we gotta look at what the test is looking for).
An ABNORMAL test detecting PrCa means its NOT A POSITIVE: ie: 35 out of 50 were False Positives
An ABNORMAL test WITHOUT PrCa means its NOT A NEGATIVE: 20 out of 100 WITHOUT PrCa were FALSE NEGATIVE.
TN = 80, FP = 15. Specificity: TN/(TN+FP); 80/(80+15) = 84%. And since it says "best represents" then 84% is closest to 80%.
20 of the 100 men without prostate cancer have abnormal test results.
Specificity = FP/FP+TN = 20/100 = 0.8 = 80%
submitted by โyb_26(316)
abnormal test result means that test detects cancer =>
35 of 50 men with prostate cancer have abnormal test result => n of pts with cancer = 50. Test shows cancer in 35 men => TP=35 => we can calculate FN = 50-35 = 15
20 of 100 men without prostate cancer have abnormal test results => FP =20 => we can calculate TN = 100-20=80
now we can calculate specificity = TN/(TN+FP) = 80/100 = 0.8 (in % will be 80%)
here is my 4/4 table: [https://www.reddit.com/r/usmlestep1/comments/ccul3w/biostat_question_from_nbme23/]