(This article was first published on

**Ecological Modelling... » R**, and kindly contributed to R-bloggers)There are many implantation in R already of ROC plots (e.g. in the packages PresenceAbsence, ROCR). I just wrote my own very simple script just to get a better understanding of it.

## ROC - plot d <- data.frame(id=1:100, ob=sample(c(1,0), 100, replace=T), m1=sample(seq(0,1,by=0.01), 100, replace=T)) # interval to calculate the threshold int <- 100 th <- seq(0,1, length=int) roc.plot <- data.frame(sen=rep(NA,int), spe=rep(NA,int)) for (i in 1:int) { # get tn, tp, fn, fp tn <- nrow(d[d[,3]<th[i]&d[,2]==0,]) fn <- nrow(d[d[,3]<th[i]&d[,2]==1,]) fp <- nrow(d[d[,3]>th[i]&d[,2]==0,]) tp <- nrow(d[d[,3]>th[i]&d[,2]==1,]) # sensitivity, if sensitivty == 1, everything all positives are found roc.plot[i,'sen'] <- tp/(tp+fn) # specificity, if specificity == 1, all negatives are found roc.plot[i,'spe'] <- tn/(tn+fp) } with(roc.plot, plot(1-spe, sen, type="l"))

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