Thursday, February 16, 2012

Mean Time Between Random Encounters

This post is in response to a question someone asked me privately. 

Usually in D&D the likelyhood of a random encounter is stated in terms of a m in n chance every p turns.  For example, in B5 - The Horror on the Hill the likelihood of encountering wandering monsters was stated, "Check for wandering monsters on The Hill every three turns when the party is moving and every six turns if they are stationary (camping). If you roll a one on a ld6, one of the following types of monsters is encountered (see table below): roll a ld6 again to determine the exact creature(s)." Most adventures have some similar statement.

For planning purposes, it might be useful to know the expected amount of time between random encounters.  It's not complicated, though.  In the above statement there's a 1 in 6 chance of a random encounter every 3 or 6 turns.  That means that you expect that out of 6 trials you expect one of them to result in a random monster.  If there's a trial every 3 to 6 turns you expect a wandering monster to appear every 18 to 36 turns. 

That makes wandering monsters pretty rare.  A lot has been said about how using wandering monsters forces the players to pick their battles.  That's only true, though, if wandering monsters are common enough to be a real nuissance.

In The Keep on the Borderlands the wandering monsters statistics are phrased a little differently.  "The passage ways here are very busy, and for every 10’ distance covered by the party there is a 1 in 6 chance that they will encounter a group of goblins (see below).  Check every time the the party travels 30’ (a 3 in 6 chance) until wandering goblins are encountered.."

That means that you have 50% chance of encountering the wandering monsters in the first 30', so you will on average encounter them 15' into the dungeon.  A party that moves 90' per turn will run into them on average in less than a turn! 


  1. Keep on the Borderlands is seriously tough with random encounters! One every 60' on average is brutal.

    It seems like one of the main reasons for the differences between the examples from B2 and B5 would be the different scales of outdoor movement (on the Hill) and indoor (the passageway in the Caves of Chaos), and the larger distances involved in going between locations outdoor vs indoor. In other words, the number of turns (and thus encounter checks) between the fixed encounters on the adventure maps.

    Mind you, to work out precisely how many map squares a party would travel on average to visit all the locations on the Caves of Chaos map would be hideously complicated, certainly beyond my mathematical skills. :)

    1. It depends on how you read it. My opinion is that the way it's written there's only one party of wandering goblins in the goblin lair. Someone could read it differently though, in which case, for the "first level" of the Caves of Chaos (although really, there isn't a strict level structure to that dungeon) it would be a pretty brutal slog through the goblin lair.