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One of the things I like about the 5e SRD is how it's pretty simple mechanically. There is one core mechanic for most rolls, which is easy to remember, and it has enough depth (I feel) that you can adjust it to the ever-changing circumstances (and player ingenuity). There are 4 parts to a check:
⦁ The d20 Roll
⦁ Attribute Modifier
⦁ Proficiency Bonus
⦁ Other Modifiers
The d20 Roll
All checks use a d20 roll, but there are 3 ways you could roll the dice. A normal roll is just 1d20 and take the result. If you have Advantage then you roll 2d20 and take the best. If you have Disadvantage you roll 2d20 and take the worst. Pretty simple to remember, but does it really matter? Well, let's turn to our friend, AnyDice.com, and see...
So Advantage basically doubles your odds of getting a 20 and drops the odds of a 1 to "very rarely." Disadvantage works the same in reverse. So it's a pretty hefty change in the odds. In combat a 1 is an automatic miss and a 20 an automatic hit, so it can really matter then. Some GMs I've known have done the same with skill checks, or given some bonus on a 20 and penalty on a 1, but that's a house rule.
While some abilities give Advantage (can't think of any that impose Disadvantage off the top of my head) what I like about it is using it as a GM tool. Offering Advantage can be a nice way to reward a player for clever thinking, or to tempt a player into doing something stupid. It's important to convey to the players that this is a pretty significant bonus/ penalty (something I don't think the SRD does a good job of) though. Even a poor character, with low stats or no proficiency bonus, has a moderate chance of success if they can gain Advantage - which is something the players need to know to encourage them to think of how to generate Adv when they have to roll something they are bad at.
According to the rules any number of Adv and Dis cancel each other out. So if 3 different things gave you Dis and 1 thing gave you Adv, then you would roll normally. I'm not so fond of this. Given how little there is to mechanically effect a roll, I'm okay with having the players try to stack up bonuses to overcome penalties. That gets kinda meta-gamey, but it also works from a role-playing sense, and I think it can encourage the players to get creative and figure out how to turn a negative situation into a positive one, or can be fun when the GM turns a positive situation into a negative one :)
Attribute Modifiers
Every roll adds one attribute modifier (subtracting if it's a negative modifier of course, adding a negative number is the same as subtraction after all) (honestly though, I've seen few negative mods overall). The SRD usually lists an attribute, but you could use any if common sense says a different one would be more appropriate.
The highest you could start with is a score of 20, for a +5 modifier (if I remember right). The highest the attribute table goes up to is 30, for a +10 modifier.
Proficiency Bonus
Depending on what you're doing, you might also add your proficiency bonus from a skill, tool or saving throw. The SRD lists which skills go with which attributes, which is just stupid and should be ignored. Use whatever skill and attribute combination that makes sense, the game is too unpredictable to hard-code these things. The proficiency bonus is fixed to the character's level, though a few abilities will increase it (usually by 150%), it starts at +2 at 1st level and goes up to +6 at 17th level (which still baffles me that it doesn't hit the highest at 20th level, the pinnacle of your development).
These compare fairly evenly to the ability modifiers. Most players will have at least a +2 at their key attributes, and while the +5 is the highest possible starting modifier, it's fairly unlikely. A +2 or +3 would be the most common, +4 for that focused character (or lucky roller). So the odds of success between someone with good attributes and poor attributes is usually about a 10-15% difference. It takes a change of both attributes and proficiency to swing the odds by 20-40%. This is a big change from previous editions and some other games. The fairly flat math of the 5e SRD means that every character has a decent chance at any activity, generally speaking. It would be pretty rare to have no chance of success, especially when you add in the possibility for your GM to give you Advantage from a clever idea. Players again need to know this, and possibly be encouraged to roll on something they might see as outside their specialty.
Other Modifiers
The SRD does not explicitly say this (at least that I can remember), but it's not unreasonable to think that a GM might want to award a smaller modifier than Adv/Dis, something like a +/-2. That's only about a 10% shift, enough to add up, but not a big deal. Kind of a consolation prize, but it's still something.
Along with this, I don't remember the SRD addressing it, but it's only logical to say that some things will be automatic success or failure. That's an important thing for everybody to keep in mind. The rules are meant to encourage creating a logical and consistent world, but they are not, and cannot be, perfect and cover every situation. There will be times when the GM has to step in and say something just logically can't be done, or logically will work. It's also very, very important that the GM make this clear to the players. If the players come up with a really clever or amazing idea, let them win. Don't drag out the dice, just narrate how their awesomeness saves the day and give them all a pat on the head. That's not something you always want to do, but if you keep your eyes open you'll find those moments when the players need to be rewarded for working (and thinking) hard. Likewise, whatever the rules might say there will be times when something just should not be possible to roll, and make sure you tell the players that before they commit to a course of action.
The Check
Okay, having gone over the general outline of how to roll, let's break it down and start looking at the probabilities. Again, with the help of AnyDice.com I've got the odds of success for each of the 6 difficulties in the SRD, with Adv and Dis, from a +0 to a +16...
Okay, looking at the table what strikes me? Overall I like it. I think you get a good spread of results, Adv and Dis make a nice difference, I think it's a pretty good system overall. I'm not sure about the Easy DC, granted it would only come into effect at the lowest levels but it kind of seems like if something was this easy you should just go ahead and say the character succeeds. It's interesting that Adv/Dis has the biggest impact in the middle. If you had a 5% chance of success Adv would make that 10% (which may be double in absolute terms, but still sucks), but if you had a 45% Adv would make it 70% (going from 'iffy' to 'reliable').
From a beginning character standpoint, let's look at 3 lines: the +0 for someone with no proficiency and no attribute mod, +2 for only one of those, and +4 for both...
Modifier Very Easy (5) Easy(10) Medium(15) Hard(20)
+0 64 80 96 30 55 80 9 30 51 1 5 10
+2 81 90 99 42 65 88 16 40 64 2 15 28
+4 Win Win Win 56 75 94 25 50 75 6 25 44
So having both, with the +4 modifier (or the max starting attribute mod), is a pretty good edge on the others. You auto-succeed at the Very Easy challenges, you've got a 75% on Easy and a 50% on Medium. Get Adv on either of those and they get pretty reliable. Your 25% on a Hard sucks, but might still be enough to risk. Given that's a character with an above-average attribute and is proficient, those seem like the odds you'd expect. The character with a +2, only proficiency or a 14-15 attribute, is not as good but still not bad. With the 90% chance of a Very Easy I'd just let the player have it as a GM (unless there was a really, really bad potential consequence for failure). The 65% chance of an Easy is not great, so getting Adv is pretty important. The 40-65% on a Medium are not great at all, but worth risking if the stakes are fairly low. At a +0 modifier it's even more critical to get Advantage, getting 80% for Easy makes it pretty reliable and even the 51% Medium is again worth trying if the stakes are fairly low. For the +0 and the +2 you wouldn't want to think about a Hard challenge unless you can get Advantage and the stakes were low.
Overview
Looking at the system, it does a good job and is simple to use. Instead of chasing a million modifiers getting Advantage, or avoiding Disadvantage, becomes the focus of a challenge. That does nicely reduce a challenge to the one or two most important elements. And while the overall spread is pretty narrow, given that a +16 is the highest you could get at level 20 (or, 17 actually). In the 3.5 SRD/ Pathfinder a +40 modifier is not impossible, or even that hard, at 20th level. So I like the simpler but not simplistic system. So to speak :) Actually, after laying out all the numbers I like the resolution system even more than my first impression. This is a good mechanic, IMHO, and one I can build on. The only thing I think is missing at all is a discussion of risk. Saying how hard something is an important step, but the even more important question is what's at stake? If it's a matter of life and death, like a save or die effect, then a 25% chance is not something you want to take. But if it's a wager of a few copper pieces, or a few hit points out of hundreds, then 25% is not a big deal. No game that I know of really talks about risk though, it is a hard thing to codify given all the variables, but it's still a key part of considering a course of action.
Another thing the SRD does not address is hidden vs explicit target numbers. I'd say the general advise I've heard is to hide the numbers a player needs, but I'm actually not a big fan of that overall. I think it's a good thing for players to know what they need, it lets them make intelligent decisions, and my compromise for unpredictability is that I'll usually hide the DC for the first attempt, and then say what they need. So the first swing is blind, but after that roll (hit or miss) I'll just give out the enemy's AC. On other attempts I'll give a ballpark, like "you think it'll be a 10 or 15" since the character should be able to assess the situation to some degree that I can't communicate as well to the player. Same thing with hit points, I'll say "it looks like the enemy is down to half their HP" during a fight. Or, "you think 1 or 2 more attacks will finish them." I want the players to know the odds, so that if they see they have really bad odds they can stop and think about how to change the situation. Which is what I love about the Adv/Dis mechanic. Getting Adv can turn bad odds into decent ones, in a lot of cases, and I'll gladly give Adv to a player who can think creatively. That gives me some narrative flexibility, and anybody can get Adv on anything with a good enough reason, so that gives the players something they can mostly measure and is always an option (not necessarily easy or cheap, but there).
While the strange progression of the proficiency table looks weird to me, I have to admit that overall I think the system hangs together pretty well. I also think it would greatly benefit every player to look at these tables and get a feel for the odds. Knowing when the odds are in your favor and you should charge ahead, or when things are going against you and you need a new plan, can be critical to survival.
Okay, we've taken a pretty good look at a lot of the 5e SRD so far. I'm not sure what I want to look at next, but we'll continue our journey through the rules next week.
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