How to avoid the determinism of Passive Perception

Question

From this question,

Passive perception is exactly that, passive. It's what the PCs are always using when not actively searching for something and doesn't use a roll of the die.

However, being that it is a fixed value, it is deterministic. My party, for example, has passive perception scores of 20, 19, 16, 15, and 12. My issue is that, when building a map with traps, I'm basically determining who will see them and who won't (unless they actively look for traps). The PC with 20 passive perception will notice everything the others notice. However, I don't like this system, I'd prefer there to be some randomness in it.

• I could ask the PCs to roll a Perception Check, but that alerts the players and there's always some meta-gaming associated with it (e.g., in the next few minutes, everyone is careful entering new rooms).
• I could also just tell them to roll a d20, and I add each person's perception modifier behind the screen, but I hate unknown rolls.
• I could also just roll for all of them, all behind the screen, but that makes me roll lots of dice a lot of times.
• A friend suggested asking players to roll a small die (1d6, for example) and I'll add it (or subtract it) to passive perception, behind the screen. He argues that, since it is passive, there shouldn't be such a large range of values (1 to 20), and that a small die with negative values (like a d6 ranging from -2 to 3) fixes the issue. He also says that this allows the players to roll (less work for DM, more fun for them) and it is not a secret roll (not really a d20, so players just assume I'm taking things from random tables and what not).

So, TL;DR: what is the best way to use Passive Perception (or active Perception without triggering players) in a way that is not fully-deterministic?

Answer 1

Invert your perspective: Roll dice against passive perception!

I agree that passive perception seems odd when you consider that it's a fixed number against a fixed number. There's no chance. There's a hack you can do, however, to randomly determine success without the usability issues that you described.

In your example, you have a large party with various passive perception scores: 20, 19, 16, 15, and 12. They come across a trap, which has a fixed DC to notice it. This DC is used when the characters actively seek it out.

Lets say that the trap's DC is 15. With your example, all but one of your players automatically notice the trap. There is no drama, no chance for failure.

The game is built on the concept of using dice to determine success. Static comparisons aren't very fun, so all we have to do is use the static DC to notice as the basis of a die roll.

I propose the following process to address passive vs fixed DC situations:

1. Convert the target DC to a bonus by subtracting 12 from it (DC 15 - 12 = +3 bonus).
2. Roll a d20 and add the converted bonus against the passive score of the character.
3. Determine success (If you roll less than or equal to a character's passive score, then the character succeeds).

Note: the amount subtracted from the DC is 12 to maintain the same probability of success as an active roll! Thanks, Josh Clark for correcting me.

So, with your example, convert your trap's DC of 15 to a +3 bonus, and then roll a d20. Lets suppose you roll a total of 19. Two of your players have noticed the trap.

This doesn't have the pitfalls of any of the supplied solutions in the question:

• The players don't have to roll, so there's no playing around with metagaming (at least any further then when you roll a hidden die).
• The number of dice you need to roll is fixed.
• The probability curve isn't as intense as when five people roll perception.
• Requires little work.

There are some ramifications, however, of this: the party is basically challenged once against the best character's passive score. This could be considered a bug or a feature depending on your perspective. If you consider it a bug, you could use a variation of the group check rules found in the DMG: roll for each player's passive perception, and only allow success if half or more of them pass the check.

In practice, I've found this system to be very quick and fluid, with the given caveat that you must have all of your players' characters' passive scores pre-recorded. This is a requirement for any usage of passive scores, so I don't consider it an important point with this.

This system isn't really that much different to players than using passive scores as supplied in the DMG, since the characters will be passively using their bonuses in either case. I'm honestly not certain that my players noticed the difference when I adopted this system. For me, however, I didn't have to feel like I was condemning my players to an automatic snafu because I set a DC high.

As a final note, this post was inspired by an article by the AngryGM. Credit where credit is due.

Answer 2

There's a fine line here, because you don't want to over-penalize players who invest in passive perception, but you also don't want it to be the have-all and end-all of traps and encounters.

The way I handle it is a few-fold.

1. Avoid metagaming by training your players to only do things when appropriate.

   This can be difficult, if your players aren't cooperative, but I regularly ask for checks when there is nothing and keep track of how long they spend doing things. Thoroughly checking a room takes time, which might mean buff spells running out, enemies getting more chances to hear them or happen upon them, and a lot of other things. You need to remove the trigger/reward interaction between asking for their rolls and them thinking something is up.

2. A succesful PP check doesn't mean they know everything. Ask what they search.

   The second part of this is my favorite. A PP check might just reveal they notice "something" or "something" draws their attention. They may not know exactly what, just that it's in that square or over in that direction. When they search an area, ask them how and where they search (and be sure to note where you are putting traps and the like). This can reward them for paying attention. Say the first trap in your dungeon is located on the ceiling. If they specify they are looking at the ceiling, they are more likely to find future traps that are there. However, each additional place they are searching takes more time (see 1) so they have to be tactical about their search patterns.

3. Be nefarious

   The people who build traps in dungeons don't want them spotted and know there are people out there who look for traps in dungeons. Be tricky with them. Hide triggers in one trap that, if it is disabled, triggers a second trap (higher Perception/Investigate check to notice that trigger). Hide traps in the doorframe, so if they aren't careful opening the door, they trigger it (better if it is not visible from the other side of the door). Rig traps that are just decoys and alert guards in the dungeon something is up. Have traps that are on a delay, waiting several rounds to trigger. The sky is the limit.

Answer 3

Apply advantage/disadvantage to passive Perception.

There are rules for applying advantage and disadvantage to passive scores (page 175 in the PHB):

A passive check is a special kind of ability check that doesn't involve any die rolls.... Here's how to determine a character's total for a passive check: 10 + all modifiers that normally apply to the check. If the character has advantage on the check, add 5. For disadvantage, subtract 5. The game refers to a passive check total as a score.

For example, a character whose passive Perception score is normally 18 would have a score of 13 if they are currently suffering from disadvantage on Perception or a score of 23 if they currently benefit from advantage.

Applying advantage and disadvantage to Perception in different scenarios, especially when it applies differently among the characters, is an easy way to avoid the determinism of passive scores by ensuring that, on occasion, characters who are normally the best at Perception will be surpassed by those who are normally worse or the worst. You can apply advantage and disadvantage secretly to preserve some of the surprise or publicly so the players know what they are getting into.

Importantly, this does not require any extra rolls or homebrewing: it makes use of the existing rules for passive scores. All it requires is a quick judgment followed by some mental math.

As long as you are fair and reasonable in assessing the scenarios in which to apply advantage and disadvantage, you will not be nerfing characters who have good Perception. Rather, you will be offering variety and a chance for the other characters to shine. When possible, focus on giving advantage moreso than disadvantage (notwithstanding the Darkvision example below, which is the most common scenario). It is better to make a few players feel empowered than to make a few others feel disempowered.

Example applications of this technique.

If you have a party in which some characters have Darkvision and others don't, you can make use of dim lighting. Everybody can still see in dim lighting, but those without Darkvision have disadvantage on Perception involving sight. If the characters who usually have the best passive Perception scores don't have Darkvision, their scores will be reduced in dim lighting. Since many dungeons are dimly lit anyway, this allows those with Darkvision to pick up the slack.

Other reasons to confer disadvantage on a player's Perception might be that they are too far away from a trap to notice it, or they are too distracted at the moment by something else to notice it, or they are so unfamiliar with the nature of the trap that even though they notice it they don't actually perceive it as a threat worth paying attention to.

Some reasons to confer advantage on a player's Perception might be that they are especially close to the trap, or they are currently assigned to paying attention to traps while the others go about different business, or they have prior knowledge or experience that makes them more likely than the others to perceive the trap. You could also consider giving a character advantage if they decide to use class features or spells that should reasonably improve their chances of noticing a threat.

Note about other passive scores.

You can apply the same guidance in this answer to other passive scores, such as those for Insight, Investigation, and Stealth to prevent monotony there as well.

My experience using this technique.

I have applied this technique in most dungeons I have DM'd across several campaigns. I have found it much simpler than rolling active Perception checks on a regular basis. Generally the player with the highest passive Perception score will be somewhat surprised the first time they are not the first to notice something. Usually the only overt confusion is due to players being unaware of the rules for dim light, an unrelated issue. In either case, once players understand the rules, they tend to take the technique for granted and respect the results, so things go smoothly.

I also use this technique regularly for passive Insight scores. The results at the table are similar, except that usually players do not notice it at all because they do not typically realize that passive scores other than Perception even exist. I occasionally use this technique to modify passive Arcana, History, Nature, and Religion checks, taking into account what I know of each character's background before declaring whether somebody is intelligent enough to have known a particular piece of information. However, I don't think determinism is quite as much a problem for Intelligence-based skills.

Answer 4

Characters can have advantage or disadvantage on perception against different situations, depending on life experience

You can grant advantage (a +5 to passive perception) for the characters who are familiar with whatever there is to notice.

Some examples

One of my players had an assassin who was looking for a lycanthrope. After sneaking into the guildhall in disguise, he was able to instantly notice that one of the patrons had ordered multiple meals and was only eating the meat - the DC was below his passive perception after the passive bonus was added.

In the same group, the dragonborn fighter noticed automatically that the club used by a goblin was carved from a dragon bone due to passive advantage.

In a different group, the barbarian (native to the region in question) was flanking a village with 2 mercenaries (barbarians from a different region) and I gave her passive advantage to spot hunting traps. One of the mercenaries attempted to disarm the trap and ended up dangling on a different trap by his ankle.

In that group, I also had the sailor, who would navigate by finding 3 constellations, instantly notice a new shiny object in the sky between the Dog and the Elephant.

Advantages of this method

This method grounds characters in the world in a way that is not dependent on rolling dice. It encourages role playing becuase it relies on details that exist in the player's imagination.

Case in point, the world has now constellations, although there is no in-system reason for them.

Disadvantage

I've never had complaints, although I don't tell my players that I use this system of advantage based on culture / upbringing. Certain types of player might find it unfair, if they have 2 higher passive perception than the player who is given the passive advantage, and fails to notice - I don't think anybody has ever done the maths.

Answer 5

Great suggestions so far. But I would like to add that I enjoy misdirection as well. Allow the passive perception to notice things but let that be the method by which you set up the scene for something else to occur. The trap / thing to be noticed is not the point of the encounter/difficulty. It is the shiny thing you use as the focus of a misdirection.

Examples I have personally run:

Climactic Weather

Overland setting: Passive Perception allows the party to notice dust on the horizon or some other sign that over yonder something is up. It appears a decent sized group of unknown somethings is either tracking them or intercepting them. And will take eventually catch up to them. They make all their careful preparations, see who / what it is (in this case it was orcs herding a a bunch of cows). They make their plans to combat what they see: basically do battle prep. Meanwhile I describe that the weather steadily gets worse and worse. By the time the force (again cow-thieving orcs on horseback) overtakes the party, a seething storm is imminently inbound. The orcs bypassed the players, avoided combat, and basically stuck to their job tending herd.

The Reason? I now had the players committed to where they were… what they were doing, and what buffs and spells were prepared etc…. And that is when I threw a non-magical not-an-elemental-at-all tornado at them.

In this case, passive perception allowed me to set the scene, and get everyone on the same page without spoiling the surprise.

A room of traps and a door that wasn't a door

Players walk into a room that is like an atrium. Fake Skylights with illusion of the sun. Paving stones run through a garden of beautiful, but obviously deadly man-sized Venus flytrap–style carnivorous plants. This path ends at a locked steel reinforced wooden door. Passive perception of different levels gave them the info that:

1. The plants would not attack until you stepped in their space.
2. The far door had a poison needle trap on the lock.

Once the players have disabled the poison trap, and unlocked the door, the real trap springs. The far door opens, to a dead end: not even a full five-foot space behind it. Upon opening the far door, the paving stones shift into their true form of stone elementals — reflex saves all around or take damage; even if the players avoid the attacks they are now in threat to get attacked by the carnivorous plants. The door they came in now has sleep gas pouring out of its lock mechanism.

Here, passive perception works in our favor. The very obvious traps are super obvious. The party is lulled into a sense of meta "We got this." They don't bother to search. Their passive perception told them all they wanted to hear. The trickier and more deadly trap is their expectation of "normal."

The trap is that the traps aren't the real trap.

When used well, misdirection is powerful!

Answer 6

That's not what Passive checks are for

Passive Checks are not for when the characters are not actively doing something; they are for when the characters are actively doing something over and over and over, and no-one wants to roll hundreds of dice.

They are also for when the GM does not want to tip off the players by asking for a skill check.

Players Basic Rules v0.3, page 59.

A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

An "average result" check

"OK, can I get a WIN\Perception check for each 10 feet of wall searched please. For the next half a mile."

"Right, there are a dozen hidden foes trying to ambush the party. Can I get 12 WIS\Perception rolls from everybody please."

"Your rogue is stealthing past the entire enemy camp? Excellent. Hang on while I roll WIS\perception for every monster in the camp."

It is less work to compare the DCs of the traps or ambushers to the passive scores of the observers.

A "don't tip off the players" check

As players, we shouldn't metagame this, but we do. As soon as the GM asks, "WIS\Perception check please," we all go on high alert.

As a GM, if you think this is an issue, you can avoid it by using passive checks.

You can also avoid it by using pre-rolled checks, as noted in other answers here. I'm a fan of this approach. When the players ask, "But don't I get a roll?" I can smile and answer, "You did get a roll, you failed it. Mouahahahaha."

Answer 7

Perception will not detect the trap. The character needs to tell you exactly where they are looking in order to have a chance at detecting it. Per the "Finding a Hidden Object" sidebar (PHB p. 178):

When your character searches for a hidden object such as a secret door or a trap [...] In most cases, you need to describe exactly where you are looking in order for the DM to determine your chance of success.

The passive Perception might or might not (depending on your decision on how apparent the trap is) merely get the party to be aware that something is amiss. Detecting the trap will require an action on the player's part telling you exactly where they are looking; thus, an active Perception check roll will be required.

So your party might notice something unusual in the corridor ahead - it could be a unusually wet part of the dungeon floor - but they will not exactly know what causes it. If they stop and search further, they might, or might not, discover the trap. If they don't, it is possible they will decide to move on and trigger it.

You should also regularly have them detect suspicious stuff in the dungeon but which will turn out to be nothing dangerous (no trap). Eventually, it will keep them on their toes. They will perceive suspicious stuff but they will have to figure out if they will spend time and search the area every time or not

Answer 8

I love the idea that Bill Nadeau had to roll against the player's passive perception, especially since this retains the bonus that a player gets from taking the observant feat. But OP points out, if something is being done passively, you may want to reduce the randomness of the check.

So I propose a hybrid solution: roll a d10 with appropriate bonus vs the players passive perception.

In this case, take the trap DC and subtract 5. This becomes the bonus for your d10 die roll.

So a DC16 trap would have a bonus of 11, added to the d10.

This means that your PP20 character will still pass the DC 95% of the time, but there is a 5% chance that he misses it (unless you want to apply crit rules to the die rolls, in which case be sure to remember that a 1 is a crit success for the players, and a 10 is a crit failure).

This will give a +/- 5 range to the DC, which introduces some variation, but not so much that it's just a secret roll.

Also, feel free to use a single roll vs all your players. This is essentially a skill check by the trap setter to hide vs the PC passive perception.

You could also consider using other dice if you want a different spread. Just subtract half of the middle value and roll away! (Subtract 6 for d12, 4 for a d8, etc...)

Answer 9

Reading between the lines, you have two objectives:

1. to introduce some variability into which PCs notice hidden things - 'I want the other PCs to have a chance to notice things, rather than the PC with the highest passive Perception noticing everything by default'.

2. to introduce some uncertainty into whether the party, as a whole, notices hidden things with a static DC - 'When I place this thing (trap, secret door, hidden treasure) and set the DC for finding it, I don't want to know for sure whether the party will discover it through passive Perception'.

If I've got those wrong, what follows may not help much.

1. Variability in which PCs notice things

First of all, I believe an important constraint is that players who have invested in PCs with high Perception modifiers should still feel that their investment was worthwhile: of all the members of the party, they should still notice most things most of the time. So the variability mustn't be too great.

The simplest approach is to get away from the static nature of passive Perception: to introduce randomness, but only a little so that we satisfy the players of perceptive PCs. This is easily done by rolling a die to generate a small number to add to or subtract from the passive Perception score, eg roll a d4 and on a:

1. subtract 1
2. leave unchanged
3. leave unchanged
4. add 1

The example used a d4, but which is the right sized die to use? Again, that depends on what you want to achieve. Your goal might be to make passive Perception (PP) variable enough that the least perceptive PC can sometimes spot something the the most perceptive PC can't. Find the difference between their two passive Perception scores (eg 17 - 13 = 4). Double this to find the right sized die to use (2 x 4 = 8, so use a d8).

With a d8:

1. subtract 3 from PP
2. subtract 2 from PP
3. subtract 1 from PP
4. [no change]
5. [no change]
6. add 1 to PP
7. add 2 to PP
8. add 3 to PP

Now, if the PC with 17 PP rolls 1, their modified PP becomes 14, and if the PC with 13 PP rolls 8, their modified PP becomes 16. If the DC to find an item is 15, then the less perceptive person (at least on average) will find it while the more perceptive person misses it.

2. Uncertainty in whether the party, as a whole, notices hidden things with a static DC

In meeting the first objective, we have also met the second: in the example given above, it's quite possible for the party as a whole to miss something with a DC of 15, or to find something with a DC of 20.

Finally

I have not tried this in my game, but it seems pretty easy to apply. I would roll the modifying die behind the screen, or perhaps better still generate a set of randomly varied PP scores for each PC before the game, and tick them off one at a time as I used them.