Seriously, Rhialto, the chances of doing this would be INCREDIBLY slim-unless the town was incredibly small-and even then it is far more likely that the defenders will be defeated and/or the city's morale will be broken and they will surrender! Although I said it was a 1 in 5 chance, I can accept that the morale, improvement and unit damage would each be far more likely than population death. For the average unit, I would probably break it up as:
Civilian Death: 1 in 10.
Improvement Destruction: 2 in 10.
Unit Damage: 3 in 10.
Morale Damage: 4 in 10.
As I said in my previous post, some units might be better at doing one over the other. So, as an example, a catapult might have a 4 in 10 chance of causing improvement damage, 2 in 10 chance of damaging units, 2 in 10 chance of killing civilians and 2 in 10 chance of damaging morale. Also, though, you must remember that improvement destruction and civilian death can also lead to morale loss indirectly, so making the city much more likely to surrender!
Please understand, Rhialto, I am still nutting things out as I go, but I DO believe their should be a difference between a city and its population!
Yours,
Aussie_Lurker.