I think its covered here:
http://dom3.servegame.com/wiki/User:...fect_unrest.3F
"Basically, every patroller will catch 0,5 brigand, but the graph is linear (with 30 patrollers you have the same chance to catch, say, 0, 10, 20 or 30 brigands). So, its much more random than if each patroller has 50% chance to succeed."
It also has the calculation to compare patrollers:
"Faster units are better patrollers: an unit with a tactical (battlefield) move of 20 (cavalry) count as 1,3 foot patroller. Flyers count as 2 patrollers. Some units, like the Forester (Man) have a bonus. Mindless units have a penalty."