5e-ifying AD&D2e with some math! Ability checks.

[dreast]

One of my favorite things about 5e, my favorite edition of D&D ever, is that it's basically AD&D 2e split through a lens of good things from 3e, 3.5e, and (yes, even a little) 4e, with some numbers flipped around. Before 5e, AD&D 2e was my favorite, and consequentially I have a lot of old AD&D2e modules. I've discovered that running 5e games from the original books makes for some very fun games, once you cut out the monster stat blocks and cut the magic item bonuses (+2, +3, etc.) in half (with a little extra adjudication on certain special things, like making "third level thief" NPCs 5e "spies" instead, and specialized garrote attacks, to borrow from Night Below's early chapters). Mind, I have a lot of fun doing monster conversions in my spare time, too (although reverse-engineering 5e's system is a bit of a hassle, so I recommend you look up the subject if you want to do that yourself), but usually you can just use a 5e monster and slap another name and picture over it if you want to go fast. But something occurred to me: 5e players are more powerful in combat (which makes sense, as a lot of AD&D2e adventures were a little... overtuned, perhaps?), but what's the difference in power for the ability checks?

Assumptions: When an AD&D 2e module calls for an ability check, we give it a default DC of 15. If the module modifies it with a bonus or penalty, we slap that (in the opposite direction) on the DC in response. If the penalty or bonus is situational, use advantage or disadvantage instead.

2e: An optional rule in 2e, you make an ability check by rolling a d20 and meeting or going under your ability score, which can range from 3 to 18, but which usually maxes out at around 15 in your primary (unless you got LUCKY) and has a high chance of being about 10 in non-primary. Proficiency in a skill means you can make checks that require specialized training, whereas those checks are impossible for nonproficient characters to even attempt.

5e: Let's look at the second tier of play (the tier where 5e play is most "balanced"), so a proficiency bonus of +3, and we can assume high scores of 16-18 and middling scores of 12-14, with weak scores of 8-10. As in AD&D 2e, proficiency allows for checks requiring specialized training as well, although these are less common in straight 5e content. (I often inherit this rule in older 2e modules, since the checks are more often for "goodies" and not crucial for advancement, although the only path for advancement should never lie behind an ability check anyway).

Sample AD&D2e character: 15 "Strong", 10 "Weak"
Sample 5e character: 18 "Strong", 10 "Weak"

Example 2e situation: "A successful Spellcraft proficiency check enables a PC to tell that these are material components for evocation spells from 1st to 4th level." (Night Below, p. 10).

5e conversion: A DC 15 Intelligence (Arcana) check for the same result.

Chance of success:
Strong, Proficient:
AD&D2e: 75%
5e: 65%

Strong, Nonproficient:
AD&D2e: 0%
5e: 50%

Weak, Proficient:
AD&D2e: 50%
5e: 45%

Weak, Nonproficient:
AD&D2e: 0%
5e: 30%

Not too far off! Let's try another one.

Climbing The Great Rock Dale requires a Dexterity check with a +2 penalty for wearing metal armor (ibid, page 54).

5e conversion: The DM can rule a DC 15 Strength (Athletics) check, with disadvantage for medium or heavy armor. Note that the AD&D2e version has no proficiency gate for this one.

Metal Armor:

Strong, Proficient:
AD&D2e: 65%
5e: 42.25%

Strong, Nonproficient:
AD&D2e: 65%
5e: 25%

Weak, Proficient:
AD&D2e: 40%
5e: 20.25%

Weak, Nonproficient:
AD&D2e: 40%
5e: 9%

Nonmetal armor:
Strong, Proficient:
AD&D 2e: 75%
5e: 65%

Strong, Noproficient:
AD&D 2e: 75%
5e: 50%

Weak, Proficient:
AD&D 2e: 50%
5e: 45%

Weak, Nonproficient:
AD&D 2e: 50%
5e: 30%


Conclusion:

Using this rule of thumb for skill checks makes 5e pretty close to AD&D2e, with extraordinary characters actually having a slightly less chance of succeeding (yay caps!), but without proficiency gates, PARTIES have a better chance of certain kinds of checks than individuals do. Of course, this ignores things like Guidance, etc., that may further wonkify the rolls, but I think it's pretty good for a quick conversion metric. I could see an argument for making the default DC something more like 12 or 13 to make the odds more similar between the two, but that ignores things like Guidance that could swing the numbers the other way.

Thoughts?

[MaxWilson]

Conclusion:

Using this rule of thumb for skill checks makes 5e pretty close to AD&D2e, with extraordinary characters actually having a slightly less chance of succeeding (yay caps!), but without proficiency gates, PARTIES have a better chance of certain kinds of checks than individuals do. Of course, this ignores things like Guidance, etc., that may further wonkify the rolls, but I think it's pretty good for a quick conversion metric. I could see an argument for making the default DC something more like 12 or 13 to make the odds more similar between the two, but that ignores things like Guidance that could swing the numbers the other way.

Thoughts?

Maybe it's my fault, but I read your post a couple of times and didn't quite figure out what "this rule of thumb" is, except that you're defaulting to DC 15 for everything.

I'm actually going the other direction with my AD&D: switch ability checks to 3d6 (roll equal to or less than your ability score to succeed) instead of d20s. If I were trying to adapt a 5E adventure to AD&D I'd probably say that DC 15 is a straight ability check, and every +/- 5 DC is a -/+ 3 on the required ability check, but I could imagine tweaking those numbers slightly after doing some math.

The key thing is that linear (d20) ability checks just don't work very well and never have, even in AD&D. There's too much variance on the die roll and you wind up with weird results because luck, not ability, is the dominant factor. This shows itself most egregiously with psionics when a highly-skilled psionicist fails frequently on things that should be relatively easy for him.