# Why does Random Forest variable importance not sum to 100%?

The randomForest package in R has the importance() function to get both node impurity and mean premutation importance for variables. Why, when calculating mean permutation importance, do the results not sum to 100%?

Here's a simple reproducible example:

library(randomForest)
data(iris)
iris.rf <- randomForest(Species~., importance = TRUE, data = iris, ntrees=1000)
imp <- importance(iris.rf, type = 1)
sum_imp <- sum(imp)
sum_imp     # != 100


Thanks

• Why do you assume it should sum to 1? I see no reason for that belief. – Firebug Sep 8 '18 at 18:31
• – Firebug Sep 8 '18 at 21:22