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#1
05-27-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,269 Times in 881 Posts Reputation: 3279
The probability of having Alzheimer’s disease given the test is positive is...

A medical research team wished to evaluate a proposed screening test for Alzheimer’s disease. The test was given to a random sample of 450 patients with Alzheimer’s disease and an independent random sample of 500 patients without symptoms of the disease. The two samples were drawn from populations of subjects who were 65 years of age or older. The results are as follows:

twobytwo.bmp

Assume that about 11.3 percent of the US population aged 65 and over have Alzheimer’s disease.

The probability of having Alzheimer’s disease given the test is positive is...
 The above post was thanked by: Claus_CU (05-28-2011)

#2
05-27-2011
 USMLE Forums Master Steps History: Step 1 Only Posts: 567 Threads: 25 Thanked 524 Times in 263 Posts Reputation: 534

I think, in this question we are dealing with PPV,

PPV = Giving a test positive, the probability of having a Disease.

which taking the numbers could be like this

PPV = TP / TP+FP = 436/441 = 0.98 = 98%

Giving us the value of the people here will have Alzheimer dx.
 The above post was thanked by: bebix (05-27-2011), USMLE-Syndrome (05-27-2011)
#3
05-27-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,269 Times in 881 Posts Reputation: 3279

Quote:
 Originally Posted by rulz I think, in this question we are dealing with PPV, PPV = Giving a test positive, the probability of having a Disease. which taking the numbers could be like this PPV = TP / TP+FP = 436/441 = 0.98 = 98% Giving us the value of the people here will have Alzheimer dx.
Small Hint: The design of the study is such that a sample of diseased subjects is drawn and a separate sample of non‐diseased subjects is drawn. The researcher is not allowing for the number of diseased to arise from random sampling, but rather determining how many will be in the study. Thus, the probability of disease cannot be estimated from this study design and the prevalence rate must be used.

#4
05-27-2011
 USMLE Forums Guru Steps History: CK Only Posts: 475 Threads: 39 Thanked 353 Times in 169 Posts Reputation: 376
PPV-good one!

Sensitivity = 0.96, Specificity= 0.99, Prevalence 11%, PPV 54%

Here is the formula! Thanks. Good question!

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 The above post was thanked by: bebix (05-27-2011), Claus_CU (05-27-2011)
#5
05-27-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,269 Times in 881 Posts Reputation: 3279

Quote:
 Originally Posted by 1TA2B Sensitivity = 0.96, Specificity= 0.99, Prevalence 11%, PPV 54% Here is the formula! Thanks. Good question!
The formula is correct!!!!...but the answer is wrong.
#6
05-27-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 89 Times in 61 Posts Reputation: 99

PPV=92%?

I used the above formula:

(0.96*0.11)/((0.96*0.11)+((1-0.99)*(1-0.11)))
 The above post was thanked by: bebix (05-27-2011)
#7
05-27-2011
 USMLE Forums Guru Steps History: CK Only Posts: 475 Threads: 39 Thanked 353 Times in 169 Posts Reputation: 376

Quote:
 Originally Posted by bebix The formula is correct!!!!...but the answer is wrong.
Hi Bebix,

This time, I used the exact prevelance 11.3%, and 3 digits!

So unfair if this question appears in real exam! It takes ages even with the help from calculator! The figure should be a lot easier!
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 The above post was thanked by: bebix (05-27-2011)
#8
05-27-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,269 Times in 881 Posts Reputation: 3279

Quote:
 Originally Posted by 1TA2B Hi Bebix, This time, I used the exact prevelance 11.3%, and 3 digits! Answer is 55.3%. Thanks So unfair if this question appears in real exam! It takes ages even with the help from calculator! The figure should be a lot easier!

You don´t have to use 11.3 or 3 digits precision...just the formula
#9
05-27-2011
 USMLE Forums Guru Steps History: CK Only Posts: 475 Threads: 39 Thanked 353 Times in 169 Posts Reputation: 376

Quote:
 Originally Posted by Claus_CU PPV=92%? I used the above formula: (0.96*0.11)/((0.96*0.11)+((1-0.99)*(1-0.11)))
Wow! I need a new calculator!

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 The above post was thanked by: bebix (05-27-2011)
#10
05-27-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,269 Times in 881 Posts Reputation: 3279

Quote:
 Originally Posted by 1TA2B Wow! I need a new calculator! Your answer sounds so right!
hahaha...indeed
#11
05-27-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 89 Times in 61 Posts Reputation: 99

Quote:
 Originally Posted by Claus_CU PPV=92%? I used the above formula: (0.96*0.11)/((0.96*0.11)+((1-0.99)*(1-0.11)))
#12
05-27-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,269 Times in 881 Posts Reputation: 3279

Quote:
 Originally Posted by Claus_CU so, the answer is 92%???
yes Claus_CU

 Tags Biostatistics-Epidemiology, Step-2-Questions

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