There are a lot of RPGs out there, and each one takes a different approach to its core dice rolling mechanics. From those that want no dice rolls to super-crunch rules, it can be intimidating as a GM to pick up a new book and learn those rules for a one-shot or even a campaign. However, RPGs (and board games too) universally share the same base mechanics, with the dice being the major differentiating factor. If you can identify the dice rolls you need and your roll as GM at the table, it is a lot easier to speed through the rules and get to play the game.
Rolling 1 Die
Examples: Pathfinder, D&D, Gumshoe
This mechanic is one where you have to roll a die to determine the outcome of the encounter. Sure, the next roll may have more dice or less, but the core mechanic depends on that one die roll. Each side of the die has the same probability as any other (i.e. d20 has a 5% chance to roll a 1 or a 20). There is no real probabilistic average, since the likelihood of rolling each number is equal.
Pros: It’s simple to do, since you have a die roll, add a modifier, and do some basic math. You only have 1 die to roll, so you can keep track of your result and even use simple random number generators to simulate result.
Cons: Since each result is equal, you do not have a real ‘average’ die roll. Extreme success is as equally likely as extreme failure, and most games consider the highest roll an automatic success and the lowest roll an automatic failure.
Opinion: This is a great mechanic for a lot of games, since it’s easy to learn and run with minimal math. However, having a ‘passive’ score on any single roll is not truly representative of the potential roll you would get.
Examples: Delta Green, Outbreak: Undead
A very common mechanic is to have a percent success number on your character sheet you need to either roll over or under. Depending on either system, you modify the appropriate stats on the character sheet to make it more likely of success.
Pros: Percentiles gives a wider spread than the 1 die method, since each % is quantified instead of lumped together in increments of 5% or 10%. Also, because it is a success/failure roll, it is possible to estimate better the chance of success and modifying that roll to become better or worse where appropriate.
Cons: Once again, the probability does not entirely work this way, and augmenting a % roll can be tricky. What if a character gets their stats to 100% success? How does the GM deal with such a character who does not need to roll? You also have to evaluate how well a character does on such rolls (i.e. if you have a 60% is there a difference between rolling 2% under and 50% under?).
Opinion: This is a nicer system that expands the 1 roll die mechanic, but has much of the same issues. However, because it is a % roll, you can simplify with random number generators and assign simple modifiers without worrying too much about how unbalancing those things can become.
Examples: Burning Wheel, World of Darkness
Some systems have players and GMs roll multiple of the same dice with a target number for success. You may need to roll over a certain number, than add together the number of dice that succeeded. This introduces a much boarder probability curve to the process, and can even introduce tasks that are nearly impossible to characters without making it actually impossible (i.e. if you roll the highest number on the die you still succeed even if the target number is impossible).
Pros: It gives a better average die roll, since you know roughly how often each die will succeed and how many dice you will roll for a given action. Most of these games also possess a mechanic where characters can exert themselves to get more dice for a roll and improve their chances of success.
Cons: Logistically, this is a lot of dice to manage, and also results in maxing a single group of stats and using those exclusively so you roll the maximum number of dice all the time. That’s similar to other games, however you must manage more dice at these tables when it happens.
Opinion: Better modeling of probability in games is always a good thing. If the number of dice don’t get out of hand, it helps GMs a lot to see how good a character is by seeing how many dice the player has on a roll.
Examples: FATE, Iron Kingdoms RPG
These are systems that require multiple dice be rolled then added together to determine the base number before modifiers. Often they will form bell curves to specific results that a GM can use as an average result in planning their game. Single dice and % games use the mean (lowest + highest /2) to simulate this but it doesn’t make that the average die roll, only the average of the possible results. Here, you have a die roll that will happen more frequently and can be planned around.
Pros: Follows statistics (mean, standard deviation, etc.) so a GM can plan what to expect from their players as the game progresses. If a player plans appropriately, they can move their mean forwards or backwards, making their character unique.
Cons: Any modifier to a dice roll can have a disproportionate effect. In a d20 game, a +1 is a 5% shift. In a 2d6 game a +1 can have a 10% or more shift and even eliminate possible failure in some cases. When using such a system, having failure and success conditions becomes very important, as is balancing bonuses.
Opinion: From a behavioral standpoint, this is a very good die roll system. It models averages better and makes small bonuses very important. However, mechanics need to be very carefully weighted to make sure a character does not get out of control and skew their personal dice rolling curve too much.
Examples: Most RPGs (it would be easier to list the ones that don’t)
In most RPGs, the GM is an active participant in the dice rolling. They roll to see how well an NPC hits a player character, or contest player rolls. It involves a lot of dice rolling. The GM has to keep up on everything, from NPC sheets, stats, modifiers, etc. It is a lot of on the fly paperwork and math.
Pros: The GM has to keep on top of the game they are running, so they concentrate and prepare most scenarios ahead of time so they are not ‘winging it’ during the game.
Cons: The GM has to maintain more notes, and also spends a great deal of time on NPC rather than story. It is very time intensive at the table, and discourages free form GMing as modifying on the fly is not a beginner skill.
Opinion: This is a good system for most games, since the GM is just as much an active participant as the players. In some cases, though, the mechanics swap out when GM or player is an active participant or a passive one. The best example would be Saving Throws in Pathfinder/D&D. They are the only defense roll you make, while other defenses are fixed numbers.
Examples: Cypher System RPGs
In passive GM games, the GM is there strictly to guide the story and add conflict. They roll no dice actively against the characters, setting target number or successes for the players to meet. Most games can be run this way if the GM knows the average die roll the players need to aim for then adding any modifiers their NPCs have.
Pros: The GM spends less time ‘balancing’ encounters and focuses on the story aspect during the game. Combat is still an active component of the game, but since the players are rolling the dice, the GM makes them do the math and keeps a fixed number for them to beat.
Cons: Active participation of the GM can feel strange, especially when the players do not see the GM sorting dice. Most of these systems use fixed numbers for NPC stats, to if the enemy inflicts damage, it is generally a set number the character cannot resist without variability.
Opinion: This method reduces a lot of preparation time for GMs and time at the table while GMs do their characters’ math. For new GMs, this system keeps the same level of mechanics but makes it substantially easier.
Once you pick up on the core mechanic and your roll (see what I did there, it’s a pun) at the table, everything else is minutiae. Do you absolutely need to know every modifier for shooting from cover? Not really, because you roughly approximate it if the reference table is buried or the player doesn’t know. Can you wing attack and damage? These are usually straightforward, and you can design your game around avoiding making it complicated to start.
Try it out. Grab a new RPG you have been interested and see how it fits into these two criteria. Once you do, you should feel more confident in your abilities during your next game session.