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#1
05-19-2014
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 592 Threads: 92 Thanked 195 Times in 127 Posts Reputation: 205
Biostat Question UWSA1

A new screening test for disease X is positive in 250 out of 1000 affected pts and in 100 out of 1000 healthy control.screening test is applied to population where disease prevalance is 10 000/100,000.how many false positive do u expect per 100,ooo individual in population?

A. 90
b 900
c 1000
d 90,000
d 9000

hi guys, i went with this question and cannot properly do a 2x2 table, please

+DZ ......... -DZ

+.. 2500 ---- 81000
-..750 ----- 9000
10.000........90.000

THE bold part is what i did wrong
when it was turn to do the specificity of the HEALTHY (90.000 x 0.1) it came 9000 which according to what ive studied specificity corresponds to the TRUE NEGATIVE population (true negative rate, stated in FA), that is why i putted them on the truly negative, however i seem to be doing something wrong. im really weak in this,
please some kind soul help me why is it 9000 that is the FALSE POSITIVE

__________________
Will. Power. Faith. MATCHED IM 2016!

Last edited by drpisho; 05-19-2014 at 06:24 AM.

#2
05-19-2014
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 592 Threads: 92 Thanked 195 Times in 127 Posts Reputation: 205

i got it,!!!
sad that no one helped me here, but anyways finally understood it.

__________________
Will. Power. Faith. MATCHED IM 2016!
#3
05-19-2014
 USMLE Forums Addict Steps History: 1 + CK Posts: 143 Threads: 12 Thanked 59 Times in 38 Posts Reputation: 69

i got it too...& i want to share it...

A new screening test for disease X is positive in 250 out of 1000 affected pts and in 100 out of 1000 healthy control.screening test is applied to population where disease prevalance is 10 000/100,000.how many false positive do u expect per 100,ooo individual in population?

as the q asking for FP=?
we only need to calculate specificity: 100 out of 1000 healthy --> specificty = 90% & FP=10%

disease prevalance is 10 000/100,000 --> healthy people= 90,000

so FP= 10% of 90,000= 9000
 The above post was thanked by: drpisho (05-19-2014), Meli (05-20-2014), placebo (05-19-2014)

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