The Alexandrian

Untested: D20 Piggybacking

November 3rd, 2010

One of my long-standing concerns with the D20 system was the skewed probabilities of opposed group checks. For example, consider the example of a PC making a Move Silently check opposed by an NPC’s Listen check where both characters have the same skill modifier. In this scenario, a single PC attempting to sneak past a single NPC has a 50% chance of succeeding.

Compare this to a situation in which 5 PCs are attempting to sneak past 5 NPCs (with, again, all of the characters involved having the same skill modifiers). This effectively becomes a check in which the 5 PCs are rolling 5d20 and keeping the lowest result, while the NPCs trying to detect them are rolling 5d20 and keeping the highest result.

The average roll of 5d20-keep-lowest is 3. The average of 5d20-keep-highest is 17. That 14 point differential means that it’s virtually impossible for a party of characters to sneak past a group of evenly matched opponents. (And even sneaking past a single watchman is difficult as the average party roll of 3 is opposed by an average roll of 10.)

Of course, the odds are actually worse than this: A successful stealth attempt will also usually require a Hide vs. Spot check, so you need to succeed at not one but two checks at these outrageous odds. And this assumes that the PCs all keep their stealth skills maxed out (which in practice they won’t, particularly since it’s so pointless to do so).

The argument can certainly be made that this is realistic in some sense: A large group should have a tougher time sneaking past a sentry than one guy and more eyes means more people who can spot you. But I would argue that the probability skew is large enough that it creates results which are both unrealistic and undesirable.

In practice, the effects of the skew are obvious: Group stealth attempts quickly drop out of the game. When stealth is called for, it takes the form of a sole scout pushing out ahead of the rest of the group. And when the scout becomes too fragile to survive when the check finally fails, stealth stops being a part of the game altogether.

Since I’d prefer stealth to be a potentially viable tactic, a solution is called for.


DISTANCE / DISTRACTION PENALTIES: A guideline that can really help the stealther is the -1 penalty per 10 feet that is supposed to be applied on Listen and Spot checks. Keep about a hundred feet away from the guy trying to spot you and you can quickly cancel out the probability skew of the dice.

Unfortunately, these modifiers become kind of wonky, particularly when it comes to Spot checks. On the open plains, for example, the “maximum distance at which a Spot check for detecting the nearby presence of others can succeed is 6d6 x 40 feet”. The minimum distance of 240 feet, therefore, is supposed to impose a -24 penalty and the maximum distance of 1,440 feet impose a -144 penalty.

I’ve tried a few different ways of fixing these modifiers, but am currently just using an ad hoc sense of what the range of the check is.

TARGET NUMBERS: Instead of making these opposed checks, set a target number for the PC’s skill check of 10 + the NPC’s skill modifier. (This essentially halves the probabilty skew.)

GROUP CHECKS: Make only one check for each group. But what skill modifier to use? Using the average value is cutesy, but impractical at the game table. Using the lowest value still effectively takes group stealth off the table. Using the highest modifier means that everyone except the rogue ignores the stealth skills entirely and also creates issues with determining surprise.

And how big can a group be? One guy with a decent Hide check shouldn’t be able to sneak an army of ten thousand soldiers under the nose of a watchtower, but where do you draw the line?

Maybe you could limit the number of people covered by a check to equal the skill leader’s skill ranks? Or impose a -2 penalty per person in the group?

COMBINE STEALTH / PERCEPTION SKILLS: I’ve been folding Hide/Move Silently into a Stealth skill and Listen/Spot into a single Perception skill intermittently since 2002, so I wasn’t particularly surprised when both Pathfinder and 4th Edition went in the same direction. It cuts down on dice rolls and eliminates the undesireable “need to succeed twice” feature of stealth checks.

This does create some interesting oddities around trying to resolve invisibility, and while I haven’t found the perfectly elegant solution yet, this slight corner case is (in my experience) preferable to the constantly degrading effects of splitting the skills.

Using some combination of these solutions tends to mitigate the problem, but I’ve generally been unsatisfied with the hodgepodge fashion of it all. So taking my unified Stealth and Perception skills in hand, I’ve been looking for a more elegant solution.


Esoterrorists - GUMSHOEI found the roots of what I think may prove a usable mechanic in the GUMSHOE system:

When a group of characters act in concert to perform a task together, they designate one to take the lead. That character makes a simple test, spending any number of his own pool points toward the task, as usual. All other characters pay 1 point from their relevant pools in order to gain the benefits of the leader’s action. These points are not added to the leader’s die result. For every character who is unable to pay this piggybacking cost, either because he lacks pool points or does not have the ability at all, the Difficulty Number of the attempt increases by 2.

Obviously the point-spending mechanics which underlie the GUMSHOE system can’t be translated directly into the D20 system, but the basic structure of a lead character making a check onto which others could “piggyback” was inspiring.


When the whole group needs to perform a single task collectively (like sneaking past a guard or using group-climbing techniques to scale a cliff) they can make a piggybacking skill check.

(1) One character takes the lead on the check. This character makes the skill check using their normal skill modifier, just like any other skill check.

(2) Other characters can “piggyback” on the lead character’s check by succeeding on a skill check. The Piggyback DC of the check is equal to half its normal DC. (So if the leader is making a DC 30 check, the other characters must make a DC 15 check to piggyback on the check result.)

(3) The lead character can reduce the Piggyback DC by 1 for every -2 penalty they accept on their check. (They must make this decision before making the check.)

(4) The decision to piggyback on the check must be made before the leader’s check is made.

OPPOSED PIGGYBACKING CHECKS: The DC of the check is set by the lead character’s check. Just like any other piggybacking check, only characters who succeed on the piggybacking check benefit.

(To simplify the resolution, you can start by rolling only the lead characters’ checks. After you’ve determined which lead character succeeded, you can call for the necessary piggybacking checks. Anyone piggybacking on the failed check, of course, will fail no matter what their piggybacking check would have been.)

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3 Responses to “Untested: D20 Piggybacking”

  1. Justin Alexander says:


    I agree that making two checks per party member in a situation where the party needs to move together can be troublesome, with the odds not in their favor. But, like Confanity has pointed out, the game already has some options in it to help move this along. Setting the check to a single DC and the Aid another action are two examples of this. Also, people in heavier armor can take their armor off (though not favorable) or have enchantments put on it to lessen the burdens of their skill penalites. Items also can be added to help with this (ie. Tools that gives bonuses or a magic potion like invisibility).

    With that being said, I do like your solution. It’s one of those little things that moves away from the mechanical clunkiness and tries to fashion it into something that favors the players and roleplaying, which should have been the case in the rules. The only point I have against this option is that it’s still a bit clunky in the numbers department. Maybe if you did something along the lines of a fixed skill challenge system, I think that would be much easier mechanically.
    Thursday, November 04, 2010, 1:13:12 PM


    One last thing — editing? In this post we see “…Using the highest modifier [paragraph break] And how big…” You also assert that 14+16=24 and that 10+12=21, although in fairness the 10 should have been 9. I’m also not sure how 11 can be half of Caitlin’s stealth check, which if I read the example right should be 4= (((1+8)/2), rounded down).
    Thursday, November 04, 2010, 4:58:58 AM

    Also, the Piggyback DC isn’t 10, it’s 7 – Aurora took a -6 penalty to her check to lower this DC by 3. That means that Caitlin actually passed the check (and could not, in fact, fail). If Duncan rolled a 1 then he would have failed.

    Additionally, it’s not clear how any of the orcs could fail a DC11 perception check – they have +12, which means they will automatically succeed on a DC of 13 or lower.
    Friday, November 05, 2010, 11:33:23 AM

    Rewriting example to make sense:

    Aurora, Bartholomew, Caitlin, and Duncan are trying to sneak past a bunch of orcs.

    Their Stealth modifiers are +20 (Aurora), +16 (Bartholomew),+8 (Caitlin), and +5 (Duncan).

    The orc leader has Perception +12, the other orcs have +3

    Aurora takes lead on the Stealth check. Bartholomew decides to make his own check. Caitlin and Duncan decide to piggyback on Aurora’s check.

    Aurora knows Duncan is clanky in his plate armor, so she decides to take a -6 penalty on her check in order to lower the effective DC of the check by 3. She rolls an 8 and gets a total check result of (8 + 20 – 6) = 22.

    Bartholomew makes his check, rolls a 14 and gets a total check result of (14 + 16) = 30.

    When the party encounters a group of 8 orcs, the DM decides that the leader will roll a Perception check and the others will piggyback onto it. He rolls 9 and gets a result of (9 + 12) = 21.

    This is the DC for Aurora and Bartholomew’s checks – so both pass (just barely in Aurora’s case). But is she good enough to keep Caitlin and Duncan quiet for the duration?

    The Piggyback DC is 7 ((21 / 2, rounded down) – 3).

    It doesn’t matter what Caitlin rolls – she has +8 to a check with a DC of 7 and automatically succeeds.

    Duncan rolls a 1, giving him a result of (1 + 5)=6. Despite Aurora’s best efforts the clanking armour has given Duncan away and the orcs have spotted him.
    How many of the orcs? The leader does so automatically (it’s his check that Duncan has effectively failed against. The DM makes piggyback Perception checks for the others at DC 11 (one-half Aurora’s Stealth check, which set the DC of the leader’s Perception check). 5 of them make the check and spot Duncan along with their leader. The other 2 orcs fail their piggyback check and are unaware during the surprise round.
    Friday, November 05, 2010, 12:15:45 PM
    Liked by Guest


    I noted the “Stealth” substitution in Pathfinder and decided not to use it — I like the flavor and possible interesting scenarios from failing to move silently while remaining unseen. This also takes care of the difficulties not only with invisibility, but also with what to do about characters on the far side of a wall or in total darkness, or what to do about a character’s “Perception” in case of impaired senses — you can’t keep them from making any rolls at all, after all, but how much do you penalize the “Perception” of a character dazzled by bright light but not quite blinded?

    One way I’ve dealt with the Stealth/Perception two-rolls-for-one-success issue is to expand the Hide skill to cover any attempt at concealing large objects. So Hide would be used to hide oneself, or to camouflage a wagon in the brush, or to ensconce your compatriot behind the tapestry. And second, if people want to cut down on their rolls, I let them roll once and apply each of their modifiers to that roll. If the characters’ Hide and MS modifiers or Listen and Spot modifiers are the same, then it’s the same as making one Stealth-versus-Perception check, but without the requirement that all such checks use one flat system for all characters.

    (I also tend to DM RP-heavy combat-light campaigns with flimsy parties and extra skill points, so I can afford to have more skills demanding point investments; I know that part of the amalgamation was to decrease the demand for skill point investment in a given task.)
    Thursday, November 04, 2010, 4:58:48 AM


    An interesting solution, but I sense impending player complaints the first time a leader rolls a 1 on a relatively simple task. I see potential solutions already built into the d20 system, actually: taking 10 and “aid another.”

    For a standard guard, it seems logical to set the DC equal to 10 plus their modifier; they’re taking 10 most of the time. If a guard is on the alert, they start making active checks, bumping their average check result up by half a point and giving them a chance of catching something they’d otherwise have missed (and implying, when they rolled poorly, that glancing around nervously made them miss someone slipping by under their nose, etc.). That right there eliminates half the probability skew in many cases.

    Meanwhile, for concerted group action, why not let the leader make an “aid other” check for anyone else who accepts their guidance? For each five points by which they beat [10 or the DC of the task at hand, as you prefer], all subjects of the “aid other” action get a cumulative +2 to their own check. That way a screwup on the leader’s part doesn’t immediately make everyone else’s effort invalid, just not-easier. If you want to get a little more complex, you can use some other mechanic like Wisdom or Leadership to cap the possible bonus. I suspect that, despite the additional rolling and adding, this is no more arithmetically cumbersome than the halving and so on involved in the piggyback method.
    Thursday, November 04, 2010, 4:58:31 AM

    Dude, what’s with the comment system? Yes, my first attempt to post ran over 3000 characters, counting spaces, but when I use MS Word to check my character count and then re-post, it shouldn’t continue to give me the same message.
    Thursday, November 04, 2010, 4:58:14 AM

  2. Justin Alexander says:

    As a follow-up: This didn’t work at my table.

    The math seemed to work out all right in the limited testing it received, but:

    (1) As Confanity predicted, the players were consistently rebelling whenever the leader rolled a “1”. (More than happy to accept the successes, of course.)

    (2) The amount of bookkeeping it added was a disincentive for using the rules.

    I suspect it would work all right in a group that was more interested in the mechanical shortcuts for speeding up gameplay, but I have difficulty convincing the group I was testing it with to even roll their attacks and damage at the same time. Asking them to declare whether or not they were piggybacking, keep that in mind for 20 seconds, and then roll their piggyback checks if the leader succeeded proved too much.

  3. Justin Alexander says:

    Upon further reflection, I suspect the problem I encountered with this system was giving players the option of piggybacking or not piggybacking on a check, instead of just declaring, “This is a piggyback check. Who’s taking point?”

    With that methodology, you’d have a GM ruling whether a particular group check was:

    (a) Everybody is performing individually and succeeds or fails individually.

    (b) Everyone is working together to accomplish a single action collectively. (One person makes the check, with other characters providing Aid Another bonuses. There may be a limit on the number of people who render meaningful assistance on a particular project.)

    (c) Everyone is working together / assisting each other, but everyone still needs to succeed. (Piggybacking mechanics. Useful for a group sneaking past a sentry or climbing a mountain.)

    (d) Everyone is attempting the same task, but as long as one of them succeeds it’ll be fine. (Like, say, people taking turns trying to kick down a door.)

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