I understand that ROC
is drawn between tpr
and fpr
, but I am having difficulty in determining which parameters I should vary to get different tpr
/fpr
pairs.
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问题:
回答1:
I wrote this answer on a similar question.
Basicly you can increase weighting on certain classes and/or downsample other classes and/or change vote aggregating rule.
[[EDITED 13.15PM CEST 1st July 2015]] @ "the two classes are very balanced – Suryavansh"
In such case your data is balanced you should mainly go with option 3 (changing aggregation rule). In randomForest this can be accessed with cutoff parameter either at training or at predicting. In other settings you may have to yourself to extract all cross-validated votes from all trees, apply a series of rules and calculate the resulting fpr and fnr.
library(randomForest)
library(AUC)
#some balanced data generator
make.data = function(obs=5000,vars=6,noise.factor = .4) {
X = data.frame(replicate(vars,rnorm(obs)))
yValue = with(X,sin(X1*pi)+sin(X2*pi*2)^3+rnorm(obs)*noise.factor)
yClass = (yValue<median(yValue))*1
yClass = factor(yClass,labels=c("red","green"))
print(table(yClass)) #five classes, first class has 1% prevalence only
Data=data.frame(X=X,y=yClass)
}
#plot true class separation
Data = make.data()
par(mfrow=c(1,1))
plot(Data[,1:2],main="separation problem: predict red/green class",
col = c("#FF000040","#00FF0040")[as.numeric(Data$y)])
#train default RF
rf1 = randomForest(y~.,Data)
#you can choose a given threshold from this ROC plot
plot(roc(rf1$votes[,1],rf1$y),main="chose a threshold from")
#create at testData set from same generator
testData = make.data()
#predict with various cutoff's
predTable = data.frame(
trueTest = testData$y,
majorityVote = predict(rf1,testData),
#~3 times increase false red
Pred.alot.Red = factor(predict(rf1,testData,cutoff=c(.3,.1))),
#~3 times increase false green
Pred.afew.Red = factor(predict(rf1,testData,cutoff=c(.1,.3)))
)
#see confusion tables
table(predTable[,c(1,2)])/5000
majorityVote
trueTest red green
red 0.4238 0.0762
green 0.0818 0.4182
.
table(predTable[,c(1,3)])/5000
Pred.alot.Red
trueTest red green
red 0.2902 0.2098
green 0.0158 0.4842
.
table(predTable[,c(1,4)])/5000
Pred.afew.Red
trueTest red green
red 0.4848 0.0152
green 0.2088 0.2912
.