I have mostly used the Milestone system in the past. Therefore, my "XP muscles" might be a bit on the flabby side.

However, I've come around back to XP for the current campaign, for a number of reasons.

In setting up a matrix of XP rewards for my use ( Party overcame a Combat whose opponents CR fell into the "Medium" range, the get 150 XP @ Level 3 ), it suddenly struck me that I'm not sure how you structure the XP for an opposed social encounters.

Put the Party a courtroom with the Baron of Thessaly and have them fight to the death, then yeah - I can figure out the XP, not problem. Have the Baron make an accusation, and argue against the Party before the King and the Court ... now I'm not so sure! 

How do I judge whether or not arguing against the Baron is Hard, Medium, or Deadly?

Can do passive social skills like Perception and Insight map analogously to AC? Somehow?

Do active Persuasion, Deception, and Intimidation map somehow to attacks?

How do you handle the "difficulty" of social encounters?

When setting up a social encounter I like to use a DC style approach, which could lend itself to an XP calculation.

We start with a 10, this is a person who is neutral to the party/person. Then we take into account their position in the encounter, are they pro party or con party. We start to give a +/- X per con/pro. The party is accused of something, definitely a con, how bad is the accusation, increase the modifier. Does the party have the favor of anyone in the court? Modify the DC up or down accordingly. Are there racial biases, are there political biases, whatever else may adjust the DC before the encounter begins. 

Lets say that they're accused of murder (-5), they have a Half-Orc in the party which has a negative racial connotation (-2), they just finished ridding the area of a group of bandits (+3), finally the magistrate has a political motive which will be aided by the party being the focus of the town's ire (+3) . We have a result of a DC 17 modifier to any rolls made with insight, persuasion, intimidation, etc.

From here the players will present their arguments and evidence of innocence. Each successful roll or irrefutable piece of evidence reduces the DC by 1, more if they provide something of great significance. As the encounter starts up you make sure the players know there is a time limit, and within that time limit their goal is to prove their innocence in any manner they can (reduce the DC to under 10). Once the time is up, or they succeed, the encounter is over. 

Maybe you can use this for what you're attempting, I've found it makes for some interesting play. 

It may be lazy, but I worked out the average xp awarded per session. If I have a session that is pretty much all RP, I award the average to everyone, then bonuses to people who have excelled in some way.

@MajorPuddles - yeah, that'd work, but that's really just one step removed from Milestones. I'm not saying it's a bad approach, just kind of out of the realm of a pure XP based approach.

@DMThac0 - yeah, what you're describing is something somewhat akin to a system I use, which I kind of cribbed from TheAngyDM - systemic InterACTION!

If I was going to try and elaborate a bit, I might set it up like this.

Party: Wants to convince the king to send forces to help the Kingdom of Dongor against an orc incursion ( yep, stealing Angry's example outright ).

King:

• Believes Orcs aren’t a threat: -2, DC 13 ( firm belief )
• Has Been Offended by Dongor: -3, DC 14 ( strong personal aspect )
• Enticed by the possibility of personal glory in battled: +1, DC 8 ( vain )

 I've elaborated on Angry's system here by adding DCs based on the resistance of the belief to change. The King is prideful, so it's tough to convince them to go to Dongor's aid - but he's susceptible to flattery, so it's easy to leverage that. Rolling over the DC allows you to reduce the objection. Rolling over the DC by 5 or more, allows you to reduce the objection, or raise the incentive by 2. When you deplete all the objections - or counter-balance then with incentives, you have convinced your target. If you hit a threshold negative score ( -5? ), you lose

If the Baron is counter-arguing with you, those objection scores can go up, or your incentive scores can go down.

You could - theoretically - add up the successes at each DC level:  2 @ 17, 2 @ 14 = 34+28 = 62 "success points". Note that if the Baron piled on counter-arguments, you'd require more successes, which would drive the final score up. Also note, that you could pile on the "personal glory in battle" angle, and wrack up enough incentives to overcome the objections - but as this is an easy angle, it's less effort, and thus contributes less to your final score.

Describing it out this way, this looks kind of like a 4e skill challenge!

Having a look at this article on 5e skill challenges, we might have the inkling of an XP system.

Let's keep the complexity rating 1-4, and equate these to Easy, Medium, Hard, and Deadly level encounters.

These equate to 4,5,8,10 required successes, and let's say the average DC we expect is 10. Multiplying this out, we would need 40,50,80, and 100 "success points" to qualify for each level, with an average XP for a party of level 5, to be 250,500,750, and 1,000 XP respectively.

This means, in the above example with the King, if they overcame his objections, this would come out to 62 "success points", which would class it as as Medium social encounter, worth 750 XP awarded to the Party.

If they super-stroked his ego, they might overcome all 4 points of objection with incentives for personal glory - but that would only equate to 32 "success points", which means it's an Easy encounter, only worth 250XP.

What do you think?

I think the shape might be here - I think this system could work - but I'm not convinced the numbers are tuned correctly yet.

The way I handle this is to assign the social encounter a difficulty in the same way you would a combat encounter. I decide if the social encounter is easy, medium, hard, or deadly, and then I reward the standard amount of XP for an encounter of that difficulty if the party succeeds. If they partially succeed, I reward half that or less. And if they fail, they get nothing. The exception is if the social encounter is something the PCs initiate as a way to avoid a combat encounter; in that case, I lean towards awarding the full XP for that combat encounter, unless negotiating is significantly easier than fighting would be.

Deciding the difficulty isn't quite as straightforward, but you can use similar criteria to what you would with a combat encounter.

• What resources (money for bribes, social capital, etc.) might the party need to spend?
• What are the potential consequences if the PCs fail? What are the stakes?
• What is the average DC of an ability check for this social encounter?
• How many successes are required? How many failures are allowed?

The way to see if your assessment is correct is to play out the social encounter. If it's easier or harder than expected, then that should quickly become obvious, and you can adjust the XP appropriately.

I agree - but what I'm trying to hit upon is how to gauge the Easy/Medium/Hard/Deadly rating of a social encounter - which is the part you appear to be struggling with as well.

Trying to gauge it "after the fact" might work - but one would have to gauge how the Players did vs. their rolls - i.e. Was this really harder than expected, or did the Player just have bad luck? We don't scale combat encounter XP based on whether or not the Players - or their opponents - are having good or bad luck with their rolls.

It shouldn't matter if it was harder or easier than expected: did they stream-roll over a Deadly level social encounter through amazing luck of the dice, or was it really just an Easy level encounter?

Re-thinking what I had above, I don't think what the Player rolls are is so important as to what they need. 

So each time they reduce a DC 13 objections, they get 13 "points".

In the above example, if they overcome all the objections, that should be 54 "points". If they just stroke the King's ego until he agrees so that he can aggrandize himself, that's 32 points.

That's not too far of what you highlight in your bullet points - it takes into consideration the average DC and the number of successes they need. 

However, I really like you calling out the cost in resources, and the possible stakes. I think some sort of point factor could be worked out for that.

How these points translate into encounter difficulty, though - that's the tricky part.

Eventually, I'll probably just "spitball estimate" encounter values, but I'd like to do the math through a handful of examples to make sure what my intuition is saying is at least in the ballpark.

 As for bypassing a combat scenario - I have a rule of thumb where bypassing a challenge is worth 50% of the encounter value. The idea here being that if they sneak by the Palace guards, they get 50% of the combat value of that scenario. If they fight their way back out of the Palace against those guards, they get the other 50%, but if they sneak out through the secret passage under the moat - they still get their 50%.

I go with Thac0's approach to give a bonus or penalty to DC every time the PC's do something.
Put it into a 4e skill challenge format with failures vs successes.
The XP is derived from the level of that skill challenge/NPC Participants