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smc213
I googled the meaning of abnormal test results just to make sure.
A positive test is one in which the result of the test is abnormal; a negative test is one in which the test's result is normal.
+4
This guy on youtube does a killer job on explaining all these problems. Just does problem after problem on 2x2 tables and a few other topics. The link to part 1 is added, part two you can find on the side panel I'm sure.
Part 1
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seagull
almost.
100/120 = 83% roughly 80%
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amirmullick3
Not sure what lfsuarez and seagull above mean. Here is my explanation.
Specificity = TN/(TN+FP). This test gave 20 false positives out of 100 people, and only 15 true negatives out of 50 men.
Specificity also equals 1-FPrate, and here the FP rate seems 20% so 100%-20%=80%.
+4
yb_26
abnormal test result means pt has cancer =>
TP = 35, FN = 15 (50-35), FP=20, TN =80 (100-20) => specificity = TN/(TN+FP) = 80/100 = 0.8 (in % will be 80%)
true negatives are 80 out of 100, not 15 out of 50
+2
bulgaine
If you replace the values from the question in the table of page 257 of FA 2019, yb_26 explanation is correct. Abnormal test = patient has cancer = test +
Question says 35/50 men with prostate cancer (so all 50 have cancer) only 35 have abnormal test results, meaning that TP=35 (disease + test +) and FN= 15 (disease + test - because they do have cancer but the test was not abnormal for them ).
20/100 men without prostate cancer have abnormal test results meaning all 100 DONT have cancer but 20 show that they have cancer when its not true so FP=20 (disease - test +) and TN =80 (disease - test -)
+
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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/]