Dear all,

I would like to calculate the optimal cut off (threshold) of a test using
the Epi package. Here I am presenting some data based on the output of two
tests. I am interested in identifying the optimal cut off an its 95% CI. 

Running the ROC() function with the Epi package I obtain a nice picture that
returns what I interpret as the optimal cut off with lr.eta=0.431. would be
this the optimal cut off? Otherwise I can I calculate it? And its confidence
interval?

Thank you,

Luigi Marongiu, MSc

 

####################################

test1<-c(  0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,
0,     0,   0,   0,   0,   0,   0,   0,   1,   1,   1,   1,   1,   1,   1,
1,     1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,   1,
1,     1,   1,   1,   1,   1,   1)

 

test2<-c(  0.65662706,     0.009706075,    0.003134876,    0.049831384,
1.089876198,    0.136582023,    0.165812622,    1.316512535,    0.725196908,
6.214688703,    48.26356415,    0.004775112,    0.035568226,    0.36934994,
0.452627677,    1.001904226,    0.116876181,    0.127614619,    1.345812733,
1.487365838,    4.321874201,    0.373389928,    0.002137676,    0.019958462,
43.51470584,    10.07600936,    8.668998078,    11.2505088,     45.53510205,
49.80855616,    1.375550815,    9.798996492,    9.494694175,    23.24171357,
0.779040988,    664.0886249,    15.05400384,    2.365265177,    0.076619211,
0.692909116,    8.497272898,    15.57700003,    20.83909961,    0.833613282,
2.84624862,     0.46118499,     7.330049094,    1.612815795,    3.695709614,
17.75107595)

 

### create data frame from matrix

     my.mat<-matrix(c(test1, test2), nrow=50, byrow=FALSE)

     dimnames(my.mat)<-list(c(1:50),c("class", "test"))

     my.data<-as.data.frame(my.mat)

     attach(my.data)

 

### LOAD PACKAGE 

     library(Epi)

 

### ROC analysis

           ROC(form=my.data$class ~ my.data$test, plot="ROC", data=my.data, 

main="ROC with Epi package", MI=TRUE, MX=TRUE, PV=TRUE)

 

### REPORT: lr.eta=0.491


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