Page 1 of 2: positive predictive value

docams · Credits: 464 My Scrapbook

dear friends,
this question of biostats is bugging me from a long time .... so pls can u help me find a solution to this problem & if possible pls give me the "formulas" for solving this type of problems....
the ques is :-
THE ELISA TEST HAS A SENSITIVITY OF 95% & A SPECIFICITY OF 95% , FIND THE POSITIVE PREDICTIVE VALUE OF THE TEST IF IT IS USED TO SCREEN A COMMUNITY WITH A PREVALENCE OF AIDS OF 55

doctorgirl78 · Credits: 5514 My Scrapbook

see they have given senstivity i.e. a/(a+c)=95%
specificity d/(b+d)=95%

with prevalence of 55 out of what population???? i'll assume 1000.this is one thing you missed in the question...........

a+c=55
b+d=1000-55=945

a is 95% of a+c=52.25
b=55-a=55-52.25=2.75

d=95%of b+d=897.775
c=1000-d=48.25

you got your a,b,c,d now take out a/(a+b) for PPV and d/(c+d) for NPV........

got it???

docams · Credits: 464 My Scrapbook

hi doctorgirl78,
thanks for ur reply..... but friend pls recheck the reply u posted
"a+c=55
b+d=1000-55=945

a is 95% of a+c=52.25
b=55-a=55-52.25=2.75

d=95%of b+d=897.775
c=1000-d=48.25 "

u have applied the wrong formula in solving this question...
so if possible pls recheck it

doctorgirl78 · Credits: 5514 My Scrapbook

i checked my cals dear .may be you took the abc as wrong.........
a-TP
b-FP
c-FN
d-TN

i take test +/_ on left vertical bar and ds +/- on top horizontal with and b horizontal..

give it a thought now and let me know if it still sounds wrong

pilgrim · Credits: 957 My Scrapbook

Apply this formula

PPV =(sensitivity x prevalence)/(sensitivity x prevalence) +{(1-Specificity)x(1-prevalence)

In short
PPV= SP/SP+(1-Sp)(1-P)
S-sensitivity
P- prevalence
Sp-specificity

Fill in the formula
u'll get the answer
(95/100)x55
-----------------
(95/100)x55 + (1-95/100)(1000-55)

52.76%

[/u]

Bruno · Credits: 45896 My Scrapbook My Reading List 50 Books

Both ways are correct.

But the easiest way, which I follow (though it takes a little more time) is the one used by Doc Girl.

Just imagine the total population to be 1000 and work out.

Alteranatively, you can use the formula. But one problem witht he formula is that , you have to know it 100 % and should have no doubt inthe exam hall.

The best way is to work out one way and then check it with the next !!!

Just wondering what the Best Selling Author (Dr.Tyagi) of "Biostatistics Buster" has to say

rakhijain · Credits: 14617 My Scrapbook

in this particular ques when a+c =55 then why r we calculating the value of b as 55-a kindly explain

rkrai11 · Credits: 456 My Scrapbook

You are right rakhijain, doctgirl78 has wrongly calculated the value of b, that should be value of c.This type of example is in the 17th Ed of Park. It is easy to calculate through assuming 1000.

captainprasun · Credits: 1661 My Scrapbook

PPV=sens*Prev/sens*prev+(100-spec)(100-p)

just mug up this

draditithegreat · Credits: 1248020 My Scrapbook

cant understand.. so difficult to mug!

		Sign In New User? Sign Up Sign in to access your control panel and messenger!
				TechZone \| SpiderNevi \| HowTo? \| Scrapbook!