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#1
07-07-2012
 USMLE Forums Master Steps History: 1 + CS Posts: 675 Threads: 84 Thanked 424 Times in 206 Posts Reputation: 434
Sensitivity, Specificity, PPV, NPV

What's the relationship between specificity+sensitivity on one side, and NNV+PPV on the other side?

For example:
When you decrease PPD, what happens to the sensitivity and specificity?
When you decrease the NPD, what happens to the sensitivity and specificity?

The reason I'm asking is a few UW questions that have been bugging me.
Plus there's this diagram in FA that i always end up staring at for an hour or so, it's on the page of sensitivity, specificity, PPV, NPV.
What happens if you move the B line to the right? and what happens if you move the B line to the left?

P.S. I know i asked alot of questions but they can all be traced back to one concept

#2
07-07-2012
 USMLE Forums Master Steps History: 1 + CS Posts: 675 Threads: 84 Thanked 424 Times in 206 Posts Reputation: 434

I just found this

What is the effect of the cut-off value on sensitivity/specificity/predictive values?

I think they're talking about the same thing, i'm just not able to fully digest this concept i dont know why!
#3
07-07-2012
 USMLE Forums Master Steps History: 1 + CS Posts: 675 Threads: 84 Thanked 424 Times in 206 Posts Reputation: 434

i think i got it

B line to the right (increasing your cut-off point) means:
higher specificity and lower sensitivity
with a higher PPV (because of lower false-positives).
-So a good specificity means a higher PPV due to lower false-positives

B line to the left (decreasing your cut-off point):
lower specificity and higher sensitivity
with a higher NPV (because of lower false-negatives)
-So a good sensitivity means a higher NPV due to lower false-negatives

FA should have labeled the diagram with the TN's, TP's, FP's, and FN's. After knowing which part of the diagram is which, it all becomes very easy.

Concept down!
 The above post was thanked by: mitsua (08-14-2012), Novobiocin (07-07-2012), pradeeprox (07-07-2012)
#4
07-07-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 529 Threads: 57 Thanked 282 Times in 150 Posts Reputation: 292

Quote:
 Originally Posted by Renaissance What's the relationship between specificity+sensitivity on one side, and NNV+PPV on the other side? For example: When you decrease PPD, what happens to the sensitivity and specificity? When you decrease the NPD, what happens to the sensitivity and specificity? The reason I'm asking is a few UW questions that have been bugging me. Plus there's this diagram in FA that i always end up staring at for an hour or so, it's on the page of sensitivity, specificity, PPV, NPV. What happens if you move the B line to the right? and what happens if you move the B line to the left? P.S. I know i asked alot of questions but they can all be traced back to one concept

Lol, are you talking about that bimodal or bi-hump or something curve? I hate it too. It seems simple when i look at it but then it gets soo confusing when trying to solve a question ughhhhhhhhhhh i still dont understand why we as doctors need to know that hump.
__________________
I've become so numb

 Tags Biostatistics-Epidemiology

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