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#1
05-14-2015
 USMLE Forums Veteran Steps History: Not yet Posts: 252 Threads: 32 Thanked 40 Times in 27 Posts Reputation: 50
Another Statistics question solve this

B thalassemia major results from a homozygous genotype that leads to comple of both the b-globin chains, A study subjected 100,000 participants to an intrauterine screeting test; 87 tested positive for B thal major, and the remaining 99,913 tested negative. In 7 of those 87 cases the results were shown to be false positive, Ultimately 100 of those originally screened were found to actually have the disease, which of the following is the correct sensitivity of the intrauterine screening test.

A 8%
B 80%
C 87%
D 92%
E 99.9%

how to solve something like this ?

#2
05-14-2015
 USMLE Forums Newbie Posts: 2 Threads: 0 Thanked 0 Times in 0 Posts Reputation: 10

Should give you this 2x2

80 7
100 99k

which would mean sensitivity = 80/180 = 44%... which isn't an answer choice...

but on second glance it says "100 of those originally screened were found to have the disease."
This fact includes the 80 that we have up there... meaning the correct 2x2 is:

80 7
20 99k

aka 80/100 = 80% sensitivity.
#3
05-14-2015
 USMLE Forums Veteran Steps History: Not yet Posts: 252 Threads: 32 Thanked 40 Times in 27 Posts Reputation: 50

dang how do you know how to solve this so fast ?

tell me how to study this subject ?

Quote:
 Originally Posted by Zheng Should give you this 2x2 80 7 100 99k which would mean sensitivity = 80/180 = 44%... which isn't an answer choice... but on second glance it says "100 of those originally screened were found to have the disease." This fact includes the 80 that we have up there... meaning the correct 2x2 is: 80 7 20 99k aka 80/100 = 80% sensitivity.

#4
05-15-2015
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 122 Threads: 17 Thanked 128 Times in 39 Posts Reputation: 138

Just do a 2*2 table and fill the blanks with the info they gave u .

Total number of subjects is 100000

Total number of people tested positive 87 --- of which 80 are TP and 7 are FP

if the total number is 100000 and 87 tested positive then it means (100000-87) tested negative--- of which some are TN and others are FN

He told u that 100 people have the disease (in total from all people tested) .. These 100 are divided into 80 (TP) + 20 (FN)(100-80)

Now u can easily calculate sensitivity which is 80/100
#5
05-15-2015
 USMLE Forums Veteran Steps History: Not yet Posts: 252 Threads: 32 Thanked 40 Times in 27 Posts Reputation: 50

why is not TP 87 ???? why 80, how do make sure I do not make this errors ?

Total number of people tested positive 87 --- of which 80 are TP and 7 are FP

Quote:
 Originally Posted by spinal Just do a 2*2 table and fill the blanks with the info they gave u . Total number of subjects is 100000 Total number of people tested positive 87 --- of which 80 are TP and 7 are FP if the total number is 100000 and 87 tested positive then it means (100000-87) tested negative--- of which some are TN and others are FN He told u that 100 people have the disease (in total from all people tested) .. These 100 are divided into 80 (TP) + 20 (FN)(100-80) Now u can easily calculate sensitivity which is 80/100
#6
05-15-2015
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 122 Threads: 17 Thanked 128 Times in 39 Posts Reputation: 138

in the question stem he tells you 87 ppl tested positive.. so this the total positives. Some of them will be TP and some will be FP

He has given u the FP directly in the question as 7 so FP=all positives- FP

If u draw the table from the beginning and just fill the blanks it will be easy without even considering what is TP and what is FP

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