# What happens to the left over unpicked data in Random Forest

I believe in Random forest we pick random samples of training data with replacement. My question is there still is a possibility that we might leave some data out. What happens to that. Does it not affect the random forest training? Do we use that left over later at some point.

• A couple of points to add. First, each tree will miss about $1/e$ of the data. But then, with 1,2,3,4 trees you expect to completely ignore 37%, 14%, 5%, 2% of the data respectively. With 10 trees it's down to 0.0045%, with 100 trees $3\cdot10^{-42}$%. So for most applications, you're not missing out on much. Second, any missed samples will show up in all the out-of-bag (OOB) scoring estimates, so not necessarily completely wasted. – Ben Reiniger Apr 26 at 19:35