Action Resolution in CALL OF CTHULHU
Raymond Long's blog: 2016-Feb-13
Gaming > Action resolution

CALL OF CTHULHU is my favourite published roleplaying game despite the fact I dislike almost every part of the rules system. You succeed with a skill if you roll the skill number or less on D100.
CoC has the great virtue that you can play the game without knowing the rules. You can turn up for your first game, and every so often the GM will tell you to make a roll on a skill. This is a massive plus point for the game, but there are three big problems with the skill roll system.
1. There's no concept of difficulty. Your percentage chance of spotting something hidden is equal to your Spot Hidden skill, regardless of how big it is, how well hidden it is, or the level of cover.
2. There's no system to match skill against skill. Bob is trying to hide and Jim is trying to spot him. What happens if Bob succeeds at his Hide roll and Jim succeeds at his Spot Hidden roll, or if both fail?
3. You can't make a skill roll without having the appropriate skill. There's no Pool skill on the character sheet, so your Pool skill will be zero unless you put points into it. I've never seen a CoC character with Pool skill. But any normal human can have a go at pool, even one who's never tried. Suppose in CoC, two characters with no Pool skill decide to play pool against each other. How will the game system decide who wins? Neither can ever roll a success, so presumably neither will ever pot a ball.
CoC has another action resolution system, which matches characters' characteristics (eg. Dexterity) against each other. If two players with no Pool skill play pool, the GM could match their DEX aginst each other. But then what happens if a character with Pool skill 20 comes along? How can a character with Pool skill play a character without it? And why should DEX be taken into account for characters without Pool skill, but not for character with it?
CoC fans trumpet how simple the game system is. But it's easy to devise game systems almost as simple which lack these problems. For instance, the GM comes up with a difficulty, and the player has to roll this number or more on 3D6. If the character has a skill, it's added to the dice roll. If two characters compete, both roll and the higher roll wins. That's so simple that players could play without knowing the rules, but it's avoided all three major problems with CoC skills.
With just a smidgeon more complexity, characters also have characteristics which are zero for an average human. Playing pool, players roll 3D6 and add their DEX characteristic, plus Pool skill if they have it. Then DEX is taken into account equally whether a character has Pool skill or not, unlike CoC.
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