Monday, June 29, 2015

Odds for random encounters

JD Jarvis had an interesting post on encounters and encounter frequency. While I've always used different encounter tables based on terrain type or location, JD raised an interesting point.

JD rightly points out that terrain should not be the only variable in encounter probability. For example, a party traveling along an unforested ridgeline should be much easier to spot (potentially leading to more encounters) than a party who is carefully moving along low spots and using available cover to keep out of sight. A party that is staying hidden in a cave should have fewer encounters than one that has spread out for foraging or hunting. Bad weather - say a cold, hard rain - will cause both sentient and non-sentient creatures to find shelter and stay in it which should lessen the odds of an encounter.

While raising an interesting issue, and an issue that I have not seen addressed in game rules, sadly the article doesn't suggest a solution. Rather than trying to mechanically account for every possible factor, I think a simpler solution would be preferable. Using a few categories of odds. Something like, an encounter is:
  1. Very Likely (five times the average probability)
  2. Somewhat Likely (twice the average probability
  3. Average Likelihood (this would be the default or typical encounter probability)
  4. Somewhat Unlikely (half of the average probability)
  5. Very Unlikely (one fifth of the average probability)
The actual probabilities could change. What I want is some significant differentiation between possible states of affairs.  Next we need a list of factors to include both positive and negative.

These make an encounter more likely:

  • Terrain has high population density.
  • Party is travelling on a road or trail.
  • Party is in a settlement or urban area.
  • Party is spread out and in motion: foraging, searching, hunting, or grazing mounts.
  • Party is moving using lights at night or has a fire that is visible from a distance.
  • Party is making a lot of noise.
  • Party is using visible or detectable magic.

These make an encounter less likely:

  • Terrain has low population density.
  • Party moves during off hours.
  • Party is moving in cover and avoiding ridgelines. Movement slowed by 50%.
  • Party is staying in shelter or has posted sentries.
  • Party has no fire at night. Chance of cold or exposure.
  • Weather is adverse to travel: windy, rainy, cold, unusually hot, etc.
To keep things simple, treat each of the more likely conditions as a +1 and the less likely conditions as a -1 then total the modifiers.
3+ or more      Very Likely (five times the average probability)
+2                   Somewhat Likely (twice the average probability
-1 to +1           Average Likelihood (this would be the default or typical encounter probability)
-2                    Somewhat Unlikely (half of the average probability)
-3 or less         Very Unlikely (one fifth of the average probability)

Now we need to use an encounter probability that can scale with our five categories of likelihood. I want something that is easy to roll and understand but that scales well with the range I set. From long experience with Runequest and Call of Cthulhu I think a d100 will work well. I'll set the average probability of an encounter at 10%. That means my five categories will look like this.

Modifier         Encounter Likelihood        Probabilty
3+ or more      Very Likely                              50%
+2                    Somewhat Likely                     20%
-1 to +1           Average Likelihood                  10%
-2                    Somewhat Unlikely                  05%
-3 or less         Very Unlikely                           01%

I've intentionally left the unit of time between rolls unspecified. In an old school game of D&D random encounters are frequent. In my Honor+Intrigue game which is modeled after a mix of swashbuckling fiction and actual Early Modern Europe, many 'encounters' would be extremely mundane and pretty uninteresting e.g. small herd of dairy cows, goose girl with flock of geese, peasants walking to or from their field, etc. So there I prefer less frequent encounters. Something like 10% per day sounds about right for me.

In addition, I would treat these as guidelines not as hard and fast rules. For example, if Loud Noises like drunken singing or chopping firewood are +1, then setting off grenades, cannons, or barrels of gunpowder might be +2 or even +3 all by themselves.

EDIT: My formatting skills in Blogger really suck. Sorry. Guess I need to spend some time figuring out how the format works. Copying in from MS-Word yields inconsistent results.

UPDATE: I have a follow up post suggesting an alternate solution based on another post by JD Jarvis.

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