### 如何在邏輯回歸中得到ROC的最佳截止點作為數字

#### [英]How can I get The optimal cutoff point of the ROC in logistic regression as a number

I would like to get the optimal cut off point of the ROC in logistic regression as a number and not as two crossing curves. Using the code below I can get the plot that will show the optimal point but in some cases I just need the point as a number that I can use for other calculations. Here are the code lines:

``````library(Epi)
ROC( form = IsVIP ~ var1+var2+var3+var4+var5, plot="sp", data=vip_data )
``````

Thanks

## 1 个解决方案

### #1

11

As per documentation the optimal cut-off point is defined as the point where Sensitivity + Specificity is maximal (see `MX` argument in `?ROC`). You can get the according values as follows (see example in `?ROC`):

``````x <- rnorm(100)
z <- rnorm(100)
w <- rnorm(100)
tigol <- function(x) 1 - (1 + exp(x))^(-1)
y <- rbinom(100, 1, tigol(0.3 + 3*x + 5*z + 7*w))
rc <- ROC(form = y ~ x + z, plot="sp")
## optimal combination
opt <- which.max(rowSums(rc\$res[, c("sens", "spec")]))
## optimal cut-off point
rc\$res\$lr.eta[opt]
``````

This is the point that will be shown when you run

``````ROC(form = y ~ x + z, plot = "ROC", MX = TRUE)
``````