Helen,

I disagree in using the per capita for cities under 100k due to perception. National Guard already been to N.O back in 06 or 07 and their homicides have been mildly low compared to the mid 90s.
For cities under 100k with a mid size population or smaller doesn't need military aid. For example: My city ended last year with ( according to the per capita basis) 37 per 100k. That's higher than Chicago, NYC, & L.A. Does it need Marshall Law? No. There was only 9 homicides and half the suspects involved have been caught.

Per capita is more or less base on chances.