Today I am sharing my house rules for action tests in OSR games. These rules were originally written to replace the percentile Thief skills found in Basic Fantasy RPG and B/X D&D. They grew out of the Ability Test table in the appendix of Basic Fantasy 3rd.
Part 3: Thief Skills and Action Tests
In many OSR games Thieves get skills and no one else does. For me this doesn’t work because if my players want to try something I want to give them a chance whichever class they are playing but at low levels if I give them anything more than a 10% chance of success they will be almost as good a Thief as the actual Thief. A common suggestion is to ask players to roll under the appropriate stat on a d20 but that won’t improve as the characters gain experience and it doesn’t integrate well with most Thief skill systems, where the Thief’s player would probably rather roll against his Dex of 13 than his Pickpocket skill of 20%.
When a PC attempts a challenging task, the player must roll an Action Test. Roll a d20, the task succeeds on a 17 or higher. The player must apply the most appropriate Ability Modifier to the roll, and Thieves (or experts or similar) get a bonus equal to an equivalent level Fighter’s attack bonus, other classes gain +1 every two levels. Finally, if the activity is related to your class (e.g. tactics for a Fighter, climbing for a Thief) roll twice and use the higher roll.
So this is a d20 roll for pretty much anything not already covered by combat, spell casting, turning undead and similar. It does replace Thief skills. So use this for climbing, navigation, negotiation, perception, building stuff, breaking stuff, sailing, etc.
This rule isn’t as simple as my others but it does do a lot of work, providing a system of ability tests for all classes and balancing and simplifying the Thief’s skills.
Party Action Tests
Those times when you need to check if anyone in the party notices the strange blue slime on the ceiling so everyone rolls a Perception test and it’s pretty much given that there will always be at least one pass, so nothing can ever surprise them. Or the party are sneaking into the Lord’s manor so you need stealth checks and, thanks to the law of averages someone will always fail, which gets annoying when your players want to have a chance of occasionally getting past the guards without a fight.
Have one player roll a single Action Test for the whole party. If it is a situation where you only need to know if one character has succeeded (a Perception test or a Lore check maybe), roll 1d20 with a bonus equal to the highest experience level in the group and also the single highest relevant Ability Modifier. On the other hand, if it is a situation where you need to know if all of the PCs succeed (a Stealth test or a check to sail a large boat through a storm), roll 1d20 with a bonus equal to half the lowest experience level in the group and again the single highest relevant Ability Modifier in the party. On a roll of at least 17 the group succeeds.
Optionally, you can allow each group to have a speciality where they get to roll twice and take the highest if the Party Action Test is related to the group’s raison d’etre. So a group of mercenaries might get to roll with advantage when they are testing to spot an ambush or train a militia while a group of criminals might get advantage when rolling stealth or deception checks.
This rule can be a little complicated to explain, but it is simple enough in play and often better represents what is actually going on in the fiction. I like to rotate which player rolls group tests to keep everyone involved.