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girl^interrupted
Forum Junior Topics: 97 Posts: 746 
This is a kaplan 2009 lecture notes question. They have given the answer but haven't taken the time to explain. I just don't get it. Help me understand this plz. I'm so bad at anything remotely MATH.... Kaplan book psychiatry and biostats, page 130, question # 34: Q. 55 yr old man complains of urinary frequency. exam shows a 1 cm nodule on his prostate. physician order PSA. It comes out to be 7ng/ml ( while anything above 4ng/ml is abnormal). Using this standard , this test has a sensitivity of 80% and a specificity of 90%. recent studies show that 10% of men f this age have prostate CA. what is your best estimate of the likelihood that this man actually has prostate CA? A. 13% B. 25% C. 36% D. 47% E. 58% answer is D. 47% ( I already know the answer, I just need you to explain this answer. I just don't get it at all, the concept is not clear)  
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benstar
Forum Elite Topics: 38 Posts: 245 
this q has been explained before in the privious posts and in kaplan the q is explained on the video so maybe thats way they dont explain it anymore in the notes u must make the 2 by 2 table 10% is the prevalence of the disease always start with it when u make the table and assume u have a total of 1000 people (to make ur calculations easier but u can also put 100 or 10) disease + disease  test + 80 TP 90 FP test  20 FN 810 TN 100 900 1000 80 from Se 80% goes into TP box 90 from Sp 90 goes into FP box the rest u just substract to fill the empty boxes PPV is 80/170=47%  
belle
Forum Senior Topics: 17 Posts: 157 
hmm..I still don't get it....coz if specificity is 90%> then don't u have to put the 90 into the TN box? like TP 80 FP 10 FN 20 TN 90 confusing..  
benstar
Forum Elite Topics: 38 Posts: 245 
dont forget ur first choice to have 1000 total so after u put 80 in TP box will look like this tp 80 fp fn 20 tn T 100 900 1000 now put Sp knowing FP+TN is 900 u must put 90 in FP to have that 900 total Sp=810/900=90% this is the tricky part i know  
belle
Forum Senior Topics: 17 Posts: 157 
wow...got the concept. Thanks benstar!  
alirizvi
Forum Newbie Topics: 36 Posts: 542 
nice..  
rosetta
Forum Guru Topics: 13 Posts: 597 
wow!! thanks!!  
drnazrul
Forum Newbie Topics: 0 Posts: 2 
I dont know the current status of the person asked the question...possibly it is too late to answer...but the simplest way of answering the question is stated below You should use Bayes Theorem to solve this problem... the question ask to find out the PPV (Positive Predictive Value) of the test which is estimated by the following formula PPV=(Sensitivity x Prevalence)/[(Sensitivity x Prevalence)+(1Specificity) x (1Prevalence)] = (0.8 x .01)/[(0.8 x 0.1) + (10.9) x (10.1)] = 0.47 I hope this answers the problem. Further queries are always welcome. Cheers!  
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