It's a shame there's no true metric. If there was, I could have made my dissertation about that, though I doubt my adviser would have ever accepted that.

[Get with the times, adviser!! Ghosts are real, even if Chongyun has yet to see one.]

If it's about frequency, you could split those up into their own categories. Though splitting them by location size would be important since smaller homes would have less ground to cover compared to something like a hospital.