# Why “fitted probabilities numerically 0 or 1 occurred” is a concern?

As discussed in this post, perfect separation (algorithm did not converge) is a separate issue aside from "fitted probabilities numerically 0 or 1 occurred".

As suggested in @gung's example, the model can be perfectly fine, even fitted probabilities numerically 0 or 1 occurred.

So, why R is giving such warning? Because it might be overfitting involved?

• In my experience when R gives me this warning it means I have done something truly stupid. It is a bit like: Did you really mean ...? – mdewey May 24 '17 at 15:51
• @mdewey ye, that is exactly my question. if you check gung's example, it can be perfectly fine, why it is warning me? – hxd1011 May 24 '17 at 15:52
• Hints: what are the logits of $0$ and $1$? How would they be represented in floating point? What does that imply about the reliability of those representations? – whuber May 24 '17 at 18:19