How can I draw a ROC curve for a randomForest model with three classes in R?

Question

I'm using the R package, randomForest, to create a model that classifies into three groups.

 model = randomForest(formula = condition ~ ., data = train, ntree = 2000,      
                       mtry = bestm, importance = TRUE, proximity = TRUE) 

           Type of random forest: classification
                 Number of trees: 2000
                 No. of variables tried at each split: 3

           OOB estimate of  error rate: 5.71%

           Confusion matrix:
           lethal mock resistant class.error
 lethal        20    1         0  0.04761905
 mock           1   37         0  0.02631579
 resistant      2    0         9  0.18181818

I've tried with several libraries. For instance, with ROCR, you can't do three classifications, only two. Behold:

pred=prediction(predictions,train$condition)

Error in prediction(predictions, train$condition) : 
  Number of classes is not equal to 2.
  ROCR currently supports only evaluation of binary classification 
  tasks.

data from model$votes is looking like this:

         lethal        mock   resistant
 3   0.04514364 0.952120383 0.002735978
 89  0.32394366 0.147887324 0.528169014
 16  0.02564103 0.973009447 0.001349528
 110 0.55614973 0.433155080 0.010695187
 59  0.06685633 0.903271693 0.029871977
 43  0.13424658 0.865753425 0.000000000
 41  0.82987552 0.033195021 0.136929461
 86  0.32705249 0.468371467 0.204576043
 87  0.37704918 0.341530055 0.281420765
 ........

I can get some pretty ugly ROC plots this way using the pROC package:

predictions <- as.numeric(predict(model, test, type = 'response'))
roc.multi <- multiclass.roc(test$condition, predictions, 
                            percent=TRUE)
rs <- roc.multi[['rocs']]
plot.roc(rs[[2]])
sapply(2:length(rs),function(i) lines.roc(rs[[i]],col=i))

Those plots look like this:

There's no way to smooth those lines though, because they aren't so much of a curve as they are 4 or so points each.

I need a way to plot a nice smooth ROC curve for this model, but I can't seem to find one. Does anyone know of a good approach? Thanks very much in advance!

Answer 1

I see two problems here 1) ROC curves work for binary classifiers, so you should convert your performance evaluation in a series of binary problems. I show below how to do this. 2) You should get the probabilities of each observation to belong to each of your classes (rather than just the predicted class) when you predict your test set. This will allow you to draw nice-looking ROC curves. Here's the code

#load libraries
library(randomForest)
library(pROC)

# generate some random data
set.seed(1111)
train <- data.frame(condition = sample(c("mock", "lethal", "resist"), replace = T, size = 1000))
train$feat01 <- sapply(train$condition, (function(i){ if (i == "mock") { rnorm(n = 1, mean = 0)} else if (i == "lethal") { rnorm(n = 1, mean = 1.5)} else { rnorm(n = 1, mean = -1.5)} }))
train$feat02 <- sapply(train$condition, (function(i){ if (i == "mock") { rnorm(n = 1, mean = 0)} else if (i == "lethal") { rnorm(n = 1, mean = 1.5)} else { rnorm(n = 1, mean = -1.5)} }))
train$feat03 <- sapply(train$condition, (function(i){ if (i == "mock") { rnorm(n = 1, mean = 0)} else if (i == "lethal") { rnorm(n = 1, mean = 1.5)} else { rnorm(n = 1, mean = -1.5)} }))
head(train)

test <- data.frame(condition = sample(c("mock", "lethal", "resist"), replace = T, size = 1000))
test$feat01 <- sapply(test$condition, (function(i){ if (i == "mock") { rnorm(n = 1, mean = 0)} else if (i == "lethal") { rnorm(n = 1, mean = 1.5)} else { rnorm(n = 1, mean = -1.5)} }))
test$feat02 <- sapply(test$condition, (function(i){ if (i == "mock") { rnorm(n = 1, mean = 0)} else if (i == "lethal") { rnorm(n = 1, mean = 1.5)} else { rnorm(n = 1, mean = -1.5)} }))
test$feat03 <- sapply(test$condition, (function(i){ if (i == "mock") { rnorm(n = 1, mean = 0)} else if (i == "lethal") { rnorm(n = 1, mean = 1.5)} else { rnorm(n = 1, mean = -1.5)} }))
head(test)

Now we have some data, let's train a Random Forest model as you did

# model
model <- randomForest(formula = condition ~ ., data = train, ntree = 10, maxnodes= 100, norm.votes = F) 

Next, the model is used to predict the test data. However, you should ask for type="prob" here.

# predict test set, get probs instead of response
predictions <- as.data.frame(predict(model, test, type = "prob"))

Since you have probabilities, use them to get the most-likely class.

# predict class and then attach test class
predictions$predict <- names(predictions)[1:3][apply(predictions[,1:3], 1, which.max)]
predictions$observed <- test$condition
head(predictions)
  lethal mock resist predict observed
1    0.0  0.0    1.0  resist   resist
2    0.0  0.6    0.4    mock     mock
3    1.0  0.0    0.0  lethal     mock
4    0.0  0.0    1.0  resist   resist
5    0.0  1.0    0.0    mock     mock
6    0.7  0.3    0.0  lethal     mock

Now, let's see how to plot the ROC curves. For each class, convert the multi-class problem into a binary problem. Also, call the roc() function specifying 2 arguments: i) observed classes and ii) class probability (instead of predicted class).

# 1 ROC curve, mock vs non mock
roc.mock <- roc(ifelse(predictions$observed=="mock", "mock", "non-mock"), as.numeric(predictions$mock))
plot(roc.mock, col = "gray60")

# others
roc.lethal <- roc(ifelse(predictions$observed=="lethal", "lethal", "non-lethal"), as.numeric(predictions$mock))
roc.resist <- roc(ifelse(predictions$observed=="resist", "resist", "non-resist"), as.numeric(predictions$mock))
lines(roc.lethal, col = "blue")
lines(roc.resist, col = "red")

Done. This is the result. Of course, the more observations in your test set, the smoother your curves will be.