R help - Jun 2004 - ROC for threshold value, biometrics

Hello,

I am just a beginner of R 1.9.0.
I try to construct a predictive score for the development of liver
cancer in cirrhotic patients. So dependant variable is binanry (cancer
yes or no). Independant variables are biological data. The aim is to
find out a cut-off value which differentiate (theoratically)  from
normal to pathological state for each biological data.

How can I step in procedue to get a cut-off value (threshold) for each
variable? I think I should try by ROC. But I'm not sure. If so, someone
can lead me?
If not, someone can advice me how ?

Any advice will be cordially aprreciated.

Tin Tin Htar Myint
Research assistant
Liver unit
Jean Verdier Hospital, Bondy
France

myint.tin-tin-htar at jvr.ap-hop-paris.fr wrote:
> Hello,
> 
> I am just a beginner of R 1.9.0.
> I try to construct a predictive score for the development of liver
> cancer in cirrhotic patients. So dependant variable is binanry (cancer
> yes or no). Independant variables are biological data. The aim is to
> find out a cut-off value which differentiate (theoratically)  from
> normal to pathological state for each biological data.
A binary endpoing is not a good choice for this problem, as the time to 
diagnosis is very important.  Unless you only have a biopsy at a single 
fixed time (e.g., 5 years post study entry) it would be good to consider 
for example a Cox proportional hazards model.  And there are many 
reasons for not using a cutoff, as detailed in my book Regression 
Modeling Strategies.

> 
> How can I step in procedue to get a cut-off value (threshold) for each
> variable? I think I should try by ROC. But I'm not sure. If so, someone
> can lead me?
Besides not recommending the use of cutoffs on an overall predicted 
value, I think that using cutoffs based on separate analyses of 
predictors is even worse.

Frank
> If not, someone can advice me how ?
> 
> Any advice will be cordially aprreciated.
> 
> Tin Tin Htar Myint
> Research assistant
> Liver unit
> Jean Verdier Hospital, Bondy
> France
> 
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-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University