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Old 05-27-2011
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Stats 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...
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Old 05-27-2011
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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.
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Quote:
Originally Posted by rulz View Post
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.
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Default PPV-good one!

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


Here is the formula! Thanks. Good question!


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Originally Posted by 1TA2B View Post
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.
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PPV=92%?

I used the above formula:

(0.96*0.11)/((0.96*0.11)+((1-0.99)*(1-0.11)))
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Quote:
Originally Posted by bebix View Post
The formula is correct!!!!...but the answer is wrong.
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!
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Quote:
Originally Posted by 1TA2B View Post
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
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Originally Posted by Claus_CU View Post
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!

Your answer sounds so right!
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  #10  
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Originally Posted by 1TA2B View Post
Wow! I need a new calculator!

Your answer sounds so right!
hahaha...indeed
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Quote:
Originally Posted by Claus_CU View Post
PPV=92%?

I used the above formula:

(0.96*0.11)/((0.96*0.11)+((1-0.99)*(1-0.11)))
so, the answer is 92%???
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Originally Posted by Claus_CU View Post
so, the answer is 92%???
yes Claus_CU
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