Consider adding "Negative Predictive Value"

[Steph-Chong]

Following on from Issue #681.

There appears to be a "Negative Predictive Value" (=true negatives/(true negatives+false negatives)). I had a quick look, and I didn't notice scores already having a binary categorical metric that matches this.

Should this be added to scores? And if negative predictive value is added to scores, are there any other names by which the metric is known?

[nicholasloveday]

I have't seen this used in meteorology. Happy for it to be added. I'm not aware of any other names.