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#1
06-01-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,269 Times in 881 Posts Reputation: 3279
Biostatistics #3 - Screening test

A screening test of known sensitivity and specificity is applied to two populations. The prevalence of the disease being screened for is 10% in population A and 1% in population B. Which of the following is true?

a. The percent of all negative tests that have false-negative results is lower in population A than in population B.
b. Specificity is lower in population A than in population B.
c. Reliability is higher in population A than in population B.
d. The percent of all positive tests that have false-positive results is lower in population A than in population B.
e. Sensitivity is higher in population A than in population B.
 The above post was thanked by: Amenah (06-01-2011), Claus_CU (06-02-2011), yvk99 (06-10-2011)

#2
06-01-2011
 USMLE Forums Guru Steps History: Not yet Posts: 324 Threads: 70 Thanked 239 Times in 144 Posts Reputation: 249

The sensitivity or specificity of a test remains the same in every population ..the false positive or negative no. increase or decrease by changing the cut off value for the test.So i would go with C.
Reliability is higher in population A than B
 The above post was thanked by: bebix (06-01-2011)
#3
06-02-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 89 Times in 61 Posts Reputation: 99

C) ?
 The above post was thanked by: bebix (06-02-2011)

#4
06-02-2011
 USMLE Forums Guru Steps History: Not yet Posts: 339 Threads: 94 Thanked 331 Times in 150 Posts Reputation: 291

is it c????
 The above post was thanked by: bebix (06-02-2011)
#5
06-02-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,269 Times in 881 Posts Reputation: 3279

Sorry guys, the right answer is not C)
#6
06-02-2011
 Banned Steps History: 1 + CK Posts: 751 Threads: 133 Thanked 309 Times in 192 Posts Reputation: 319

Hello guys,
The right answer is actually D. Allow me to explain:
The sensitivity and the specificity of a test are not affected by the prevalence of a certain disease i.e false negatives and false positives respectively.
The prevalence of disease in A is higher than in B meaning that assuming we don't know the specificity and sensitivity and assuming they are both the same whether in regards to groups A or B, A will have a higher PPV; in other words, those who test positive will truly have the disease is higher than in A than in B (this is what PPV is). Since B has a lower prevalence, NPV will be higher in B than A; meaning: Of those who test negative, more of those in group B than A will be truly negative when compared to a gold standard. PPV is usually increased by a higher specificity and a higher prevalence of disease, whereas NPV is increased by increased sensitivity and a lower prevalence of disease.
Hope this helps
 The above post was thanked by: bebix (06-02-2011), chemicalboy (07-29-2013), Dr_Laura (06-02-2011), dr_ram (06-02-2011), dr.muhamad (06-02-2011), drnirajmavani (06-04-2011), excellence (05-18-2014), lokya (06-27-2013), mle guy (06-02-2011), pass7 (06-02-2011), struggle (06-02-2011), zohaib (11-28-2011)
#7
06-02-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,269 Times in 881 Posts Reputation: 3279

you're absolutely right
#8
06-04-2011
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 268 Threads: 14 Thanked 103 Times in 69 Posts Reputation: 113

Quote:
 Originally Posted by add1 Hello guys, The right answer is actually D. Allow me to explain: The sensitivity and the specificity of a test are not affected by the prevalence of a certain disease i.e false negatives and false positives respectively. The prevalence of disease in A is higher than in B meaning that assuming we don't know the specificity and sensitivity and assuming they are both the same whether in regards to groups A or B, A will have a higher PPV; in other words, those who test positive will truly have the disease is higher than in A than in B (this is what PPV is). Since B has a lower prevalence, NPV will be higher in B than A; meaning: Of those who test negative, more of those in group B than A will be truly negative when compared to a gold standard. PPV is usually increased by a higher specificity and a higher prevalence of disease, whereas NPV is increased by increased sensitivity and a lower prevalence of disease. Hope this helps
i didnt even think of PPV nd NPV ...hop the same dosnt happen in exm as time limit increases the anxiety....neways thanx...
 The above post was thanked by: bebix (06-05-2011)
#9
06-04-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,269 Times in 881 Posts Reputation: 3279

Just donīt forget this graph!

click image to enlarge
 The above post was thanked by: Al-Saoudi (06-06-2011), drnirajmavani (06-05-2011), excellence (05-18-2014), JBerger (06-05-2011), pass7 (06-04-2011), rulz (10-06-2011), singapur_md (06-04-2011)
#10
06-06-2011
 USMLE Forums Scout Steps History: CS Only Posts: 25 Threads: 0 Thanked 21 Times in 16 Posts Reputation: 31

awesome!!!!!!!!!!!!!!!!!
thanks!!!!!!!!!!!!!!!!!!!!!
 The above post was thanked by: bebix (06-06-2011)

 Tags Biostatistics-Epidemiology, Figures-, Step-1-Questions

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