[R] Fw: Logistic regresion - Interpreting (SENS) and (SPEC)

[John Sorkin]

Frank,
Perhaps I was not clear in my previous Email message. Sensitivity and specificity do tell us about the quality of a test in that given two tests the one with higher sensitivity will be better at identifying subjects who have a disease in a pool who have a disease, and the more sensitive test will be better at identifying subjects who do not have a disease in a pool of people who do not have a disease. It is true that positive predictive and negative predictive values are of greater utility to a clinician, but as you know these two measures are functions of sensitivity, specificity and disease prevalence. All other things being equal, given two tests one would select the one with greater sensitivity and specificity so in a sense they do measure the "quality" of a clinical test - but not, as I tried to explain the quality of a statistical model. 

You are of course correct that sensitivity and specificity are not truly "inherent" characteristics of a test as their values may change from population-to-population, but paretically speaking, they don't change all that much, certainly not as much as positive and negative predictive values.   

I guess we will disagree about the utility of sensitivity and specificity as simplifying concepts.

Thank you as always for your clear thoughts and stimulating comments.
John




among those subjects with a disease and the one with greater specificity will be better at indentifying  

John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)

>>> Frank E Harrell Jr <f.harrell at vanderbilt.edu> 10/13/2008 2:35 PM >>>
John Sorkin wrote:
> Jumping into a thread can be like jumping into a den of lions but here goes . . .
> Sensitivity and specificity are not designed to determine the quality of a fit (i.e. if your model is good), but rather are characteristics of a test. A test that has high sensitivity will properly identify a large portion of people with a disease (or a characteristic) of interest. A test with high specificity will properly identify large proportion of people without a disease (or characteristic) of interest. Sensitivity and specificity inform the end user about the "quality" of a test. Other metrics have been designed to determine the quality of the fit, none that I know of are completely satisfactory. The pseudo R squared is one such measure. 
> 
> For a given diagnostic test (or classification scheme), different cut-off points for identifying subject who have disease can be examined to see how they influence sensitivity and 1-specificity using ROC curves.  
> 
> I await the flames that will surely come my way
> 
> John

John this has been much debated but I fail to see how backwards 
probabilities are that helpful in judging the usefulness of a test.  Why 
not condition on what we know (the test result and other baseline 
variables) and quit conditioning on what we are trying to find out 
(disease status)?  The data collected in most studies (other than 
case-control) allow one to use logistic modeling with the correct time 
order.

Furthermore, sensitivity and specificity are not constants but vary with 
subjects' characteristics.  So they are not even useful as simplifying 
concepts.

Frank
> 
> 
> 
> 
> John David Sorkin M.D., Ph.D.
> Chief, Biostatistics and Informatics
> University of Maryland School of Medicine Division of Gerontology
> Baltimore VA Medical Center
> 10 North Greene Street
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
> (Phone) 410-605-7119
> (Fax) 410-605-7913 (Please call phone number above prior to faxing)
> 
>>>> Frank E Harrell Jr <f.harrell at vanderbilt.edu> 10/13/2008 12:27 PM >>>
> Maithili Shiva wrote:
>> Dear Mr Peter Dalgaard and Mr Dieter Menne,
>>
>> I sincerely thank you for helping me out with my problem. The thing is taht I already have calculated SENS = Gg / (Gg + Bg) = 89.97%
>> and SPEC = Bb / (Bb + Gb) = 74.38%.
>>
>> Now I have values of SENS and SPEC, which are absolute in nature. My question was how do I interpret these absolue values. How does these values help me to find out wheher my model is good.
>>
>> With regards
>>
>> Ms Maithili Shiva
> 
> I can't understand why you are interested in probabilities that are in 
> backwards time order.
> 
> Frank
> 
>> ________________________________________________________________________
>>
>>
>>
>>
>>
>>
>>> Subject: [R] Logistic regresion - Interpreting (SENS) and (SPEC)
>>> To: r-help at r-project.org 
>>> Date: Friday, October 10, 2008, 5:54 AM
>>> Hi
>>>
>>> Hi I am working on credit scoring model using logistic
>>> regression. I havd main sample of 42500 clentes and based on
>>> their status as regards to defaulted / non - defaulted, I
>>> have genereted the probability of default.
>>>
>>> I have a hold out sample of 5000 clients. I have calculated
>>> (1) No of correctly classified goods Gg, (2) No of correcly
>>> classified Bads Bg and also (3) number of wrongly classified
>>> bads (Gb) and (4) number of wrongly classified goods (Bg).
>>>
>>> My prolem is how to interpret these results? What I have
>>> arrived at are the absolute figures.
>>>


-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University

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