Re: Q use of multiple metrics

Newsgroups	sci.stat.math
Date	2021-11-28 07:43 -0800
References	<726ba82f-6f3a-4874-bf2f-74e338e8cce4n@googlegroups.com> <6ta2qg1iimbs3baqda6sh7st148uofaou0@4ax.com>
Message-ID	<adde6ee4-e43d-4e63-b48d-c920b0d6c001n@googlegroups.com> (permalink)
Subject	Re: Q use of multiple metrics
From	Bruce Weaver <bweaver@lakeheadu.ca>

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On Friday, November 26, 2021 at 1:56:03 PM UTC-5, Rich Ulrich wrote:
> On Thu, 25 Nov 2021 12:36:29 -0800 (PST), Cosine <ase...@gmail.com> 
> wrote:
> >Hi: 
> > 
> > We would use more than one metrics to test the significance of a 
> > study. The most often used ones are sensitivity (SE) and specificity 
> > (SP). However, this pair would be affected by the disease prevalence. 
> > In contrast, the positive/negative predictive values (P/NPVs) are not 
> > affected by the prevalence.
> I would not say it that way. The PPV is /based on/ the prevalence. 
> It assumes a single value for the prevalence. The rarer the condition, 
> the more likely the Positive is False. 
> 
> If you know the prevalence pretty well, you should use it for 
> your description. "Testing the significance" uses the same set 
> of numbers, same 2x2 table (I presume) for SE and SP, so you 
> don't expect more power from one than for the other. It should 
> be the same test. 
> 
> If you are interested in tests across the whole ROC curve, you 
> test the curve. If you are interested in some specific prevalence, 
> you test at that value.

The equations in this BMJ Stats Note (Altman & Bland, 1994) show how prevalence is related to PPV and NPV:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2540558/pdf/bmj00448-0038a.pdf

Note as well that nowadays, some authors are using the following terms for the predictive values:

  Predictive value of a positive test (PV+)
  Predictive value of a negative test (PV-)

I like this terminology and notation better than PPV/NPV, because it makes it clear that it is the *test result*, not the predictive value, that is either positive or negative.  

HTH.

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Q use of multiple metrics Cosine <asecant@gmail.com> - 2021-11-25 12:36 -0800
  Re: Q use of multiple metrics Rich Ulrich <rich.ulrich@comcast.net> - 2021-11-26 13:55 -0500
    Re: Q use of multiple metrics Bruce Weaver <bweaver@lakeheadu.ca> - 2021-11-28 07:43 -0800

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