Behavioral Practice question -

[ArchivalUser]

SK

Here's a question...post ur answer:

The standard test to detect an STD has a sensitivity of 90% and a specificity of 70%. If a recent cross-sectional study found that one out of every 10 people has this disease, the positive predictive value is best estimated as:

a - 21%
b - 33%
c - 50%
d - 67%
e - 85%

Every time I work it out, I get an answer that doesn't best match any of these.

[ArchivalUser]

img

25%.
What's your answer?

[ArchivalUser]

IMG

Sensetivity=true +ve/true+ve + false-ve=90/100
Sepecificity=true-ve/true-ve + false+ve=70/100
90 30 120
10 70 80
100 100
ppd=true+ve/true+ve + false+ve=90/90+30=75%
ANY ATHER SOLUTION?????

[ArchivalUser]

SK

I get TP/TP + FP = 9/(9+30)
which equals 3/13 or about 25%

This question comes from a practice Kaplan exam...any ideas?

[ArchivalUser]

Ravi


Disease
+ -
+ a b
Test - c d


Sensitivity is all about DISEASE = a
-------- = 90
a +c

Specificity is all about HEALTH = d
------ = 70
b + d


PPV = a
------
a + b


My question would be how do you get the value of b ?

[ArchivalUser]

SK

By the way, the answer key says the correct response is b - 33%

[ArchivalUser]

CC

I think it is 25%. Unless you change the specifity to 80%, then the answer is 33%

[ArchivalUser]

anna

hi sk
how did u get TP = 9.
I think img is correct.

[ArchivalUser]

SK

I used

TP = 9
FN = 1
TN = 70
FP = 30

So sensitivity = 9/(9+1) and specificity = 70/(70+30) and 10% of the people have the disease = (9+1)/(70+30)

[ArchivalUser]

SULU

Hi, Sk:
prevalence=TP+FN/TP+FN+TN+FP=10%

I got the answer 25%

What's the explanation for the answer?