Alvine Bissery <[email protected]> asks:
> I have a problem with the roctab function.
> I have 2 variables : "groupe " : 1 for disease, 0 for no disease
> "outcome" wich is a continous variable measuring
> PTH, with values from 12 to 58
> - roctab groupe outcome, d - gives :
> Detailed report of Sensitivity and Specificity
> ----------------------------------------------------------------------------
> --
> Correctly
> Cut point Sensitivity Specificity Classified LR+
> LR-
> ----------------------------------------------------------------------------
> --
> ( >= 12 ) 0.00% 100.00% 60.00%
> 1.0000
> ( >= 14 ) 8.33% 100.00% 63.33%
[...]
> How is it possible to have a specificity = 100% and sensitivity = 0% for the
> first cut point ? all people >= 12 have to be classifed has "disease people"
> because test is abnormal and so sensivity must be 100% and specificity = 0 %
One assumption of ROC is that as the classification variable increases, so
does the risk of abnormality. For your data, the risk of abnormality
decreases as the classifaction variable increases, in which case -roctab-
begins by classifying everyone as normal to begin with (>=12, for your data)
and goes from there.
Admittedly, this fact needs to be better documented.
--Bobby
[email protected]
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