I really should do some real work
Quote:
Originally Posted by
psychic
Nice Link!! Better than my back of the envelope, but gobsmacked I got it right..! lol
But wait. it then says
If the sample sizes in the positive (Disease present) and the negative (Disease absent) groups do not reflect the real prevalence of the disease, then the Positive and Negative predicted values cannot be estimated and you should ignore those values.
Alternatively, when the disease prevalence is known then the positive and negative predictive values can be calculated using the following formula's based on Bayes' theorem:
Well, I'm not taxing myself further with this Bayes Theorem thing - sounds ghastly. But does this not suggest that the PPV should abe calculated using a known wider prevalence. Presumably if the same prevalence is used, then it allows comparison of tests..?
Ducks for cover.....
Quote:
Originally Posted by
Minerbarejet
Thanks for that PT.
We await the release of further trial statistics conducted of which I believe there are a few in the offing.
One thing of note though in your offering was the line that unless the prevalence is known then PPV can largely be ignored.
This raises the question , was the expected prevalence of the group known before the trial or was the PPV established as a result of the trial.
Hmmm. Further work required
Cheers
Miner
What that means is:
The prevalence it calculates is the prevalence of the condition in the numbers you input and
if the numbers you input do not have the same prevalence as the condition in the real world then the calculated prevalence is not the prevalence in the real world.
So the prevalence in the test was what it was and the PPV for the test is what it was
BUT in the real world the prevalence may be different and thus the PPV may be different which is why the really important numbers are the sensitivity and the specificity (which are affected more by sample size).
BUT
PEB decided to throw a PPV out there.
OK - that was definitely the last post from me for a while on this subject.
Next up, on another day, will be why 100% sensitivity is only half the story and we need to talk more about specificity.
Best Wishes
Paper Tiger