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In the Sunless Citadel module, from Tales from the Yawning Portal, Passive Perception is mentioned about half a dozen times, but Passive Investigation isn't mentioned once.

However, both passives are increased by +5 by the Observant feat, and both are included when it comes to the Inquisitive rogue subclass in XGtE. In most mechanical and class mentions of Passive Perception vs. Passive Investigation, it seems they are supposed to be equals.

Passive Perception is used as the defensive stat for enemies attempting to use Stealth, but no such claim is made for Passive Investigation.

So when is Passive Investigation used, beyond being an automatic permanent minimum roll for an unmade active investigation check?


Personally, I like to use them as reactionary/defensive stats that the DM rolls against, for things that a character can miss (somebody passing by, a drop of water falling from the ceiling, knowing the intended target of a shot fired from an assassin). This is not, however, an official use of the passive skills. I'm interested in regular uses of the passive skills for the sake of this question.

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  • \$\begingroup\$ Are you asking specifically for TfYP? \$\endgroup\$ – NautArch Sep 6 '18 at 16:09
  • \$\begingroup\$ @NautArch Not specifically. I bring it up since it is an official source from Wizards of the Coast that is designed to use a wide variety of skills for new players. If Passive Investigation wasn't thrown in the face of the DM like Passive Perception was in that module, then when is it supposed to come up? I mostly use it as a reference to avoid too much RAI assumptions or homebrew suggestions. I want facts to provide to future DMs on how to improve their games for investigation-based players. \$\endgroup\$ – Daniel Zastoupil Sep 6 '18 at 16:13
  • \$\begingroup\$ What does "an automatic permanent minimum roll for an unmade active investigation check" mean? \$\endgroup\$ – Mark Wells Sep 6 '18 at 17:16
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    \$\begingroup\$ @DanielZastoupil That's true if you're an 11th-level Rogue, which strongly suggests that it's not true for everyone else. There's something wrong with this picture of how passive skills work. \$\endgroup\$ – Mark Wells Sep 6 '18 at 17:32
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    \$\begingroup\$ @MarkWells rpg.stackexchange.com/questions/48281/… Check out this question, with the highest ranking (26 point) answer. In the comments is a link to a podcast where it's explained that a passive score is effectively a "floor" or a persistent "take 10 roll", almost like the rogue 11 ability. Another answer (4 points) is a literal transcript of the same podcast with this exact explanation. And I agree with you that there's something wrong with it, but the official guy said some mostly official stuff, so some of us gotta deal. \$\endgroup\$ – Daniel Zastoupil Sep 6 '18 at 17:36
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The distinction between perception and investigation isn't that great

As I've seen it played out at table, passive Intelligence (Investigation) checks are most often called out by the DM when hidden things (secret doors, items hiding in plain sight, illusions, etc) are in the area that the PC can see or hear.

  • That said, most DM's I have played with default to Perception for a great deal of 'you notice this' more than any Intelligence-based skill. (A notable exception being our ToA DM, who uses a variety of ability based checks depending on the situation).

    Investigation. When you look around for clues and make deductions based on those clues, you make an Intelligence (Investigation) check. You might deduce the location of a hidden object, discern from the appearance of a wound what kind of weapon dealt it, or determine the weakest point in a tunnel that could cause it to collapse. (Basic Rules, p. 61).

The most common applications I have seen in game is a secret doors detection (a player had taken the Observant feat), the 'feet sticking out from under the curtains' deal (yeah, our PC spotted the thief who had run away) and a whole gamut of things that "just don't look right." Two instances stand out

  1. A 'you notice that the shadow is wrong based on where the light is coming from' as we entered a room and were examining the contents
  2. The old 'you notice a lack of a shadow' during an encounter with what was (we discovered soon after) an illusory creature.

    What I have also seen that fits very well is the use of passive Intelligence based checks that let the DM just advise one of the players "you recall (X)" without interrupting play for a roll.

    • This approach is going to be very table dependent. At some tables rolling a lot is just what is done, while at others it isn't.
    • The one I saw used most often was directed at that same player -- a passive Intelligence(Investigation) check for estimating the value of a precious item (gems and jewelry). I think the DM always told our rogue that for (1) ease/speed of play and (2) a rogue/thief would most likely be the one to pay close attention to that detail.

For a good illustration of the distinction between the two different ability checks, see @DaleM's answer.

About passive checks in general

As my friend Tanarii has suggested here, passive checks ...

... aren't for when a character isn't actively trying to do something.

They are for when a player isn't actively rolling a die. Either because they can't know a check is happening, or because it would require rolling over and over again.

Its passive on the part of the player, not the character. The character can be, even usually is, doing something active.

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    \$\begingroup\$ That last point is key. The only passive skill check actually specified in the PHB is Stealth vs. Perception, which involves using the passive skill as the DC for someone else's check. It's not a matter of either character "actively" doing something (both hiding and looking are active to some degree) but just of one person needing to roll the die. \$\endgroup\$ – Mark Wells Sep 6 '18 at 21:08
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It seems that your question is not about passive Investigation per se but is about the Investigation skill in general and how it differs from the Perception skill.

The rules make the distinction (my emphasis):

Investigation. When you look around for clues and make deductions based on those clues, you make an Intelligence (Investigation) check. You might deduce the location of a hidden object, discern from the appearance of a wound what kind of weapon dealt it, or determine the weakest point in a tunnel that could cause it to collapse. Poring through ancient scrolls in search of a hidden fragment of knowledge might also call for an Intelligence (Investigation) check.

Perception. Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings and the keenness of your senses. For example, you might try to hear a conversation through a closed door, eavesdrop under an open window, or hear monsters moving stealthily in the forest. Or you might try to spot things that are obscured or easy to miss, whether they are orcs lying in ambush on a road, thugs hiding in the shadows of an alley, or candlelight under a closed secret door.

In essence they make the distinction between understanding (Investigation) and observation (Perception). IF we imagine the Wise Cleric and the Intelligent Wizard exploring the Library of All Knowledge, the Cleric can see the disturbed dust in front of the bookcase and, when he draws it to the Wizard's attention (because she didn't see it), she can deduce the existence of the secret passage that had not occurred to the Cleric. In such circumstances it is perfectly reasonable for a DM to have a Perception DC to notice the dust and an Investigation DC to deduce the passage and/or its trigger.

Of course, circumstances exist that rely totally on observation (the determination of surprise) or understanding (recognizing the only item of any real value in a merchant's jumbled store).

Any ability check can be made passively at the DM's discretion - usually when they don't want to tip the players off that there is something going on. This doesn't mean that the character is not doing something active like looking around or fiddling with the bookshelf, just that this sort of activity can be assumed.

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  • \$\begingroup\$ Welcome back. :) The illustration of the wizard and the cleric does a nice job if clarifying the distinction. \$\endgroup\$ – KorvinStarmast Sep 7 '18 at 11:44
  • \$\begingroup\$ Part of it was the point about Investigation vs. Perception, but the passive elements also play an important role to my question. One key part of my question was trying to find where the "missing" passive investigation checks are in a campaign. Assuming both PP and PI are used the same, and PP is mentioned 6 or so times in the Sunless Citadel, and used a number of other times for stealth, where are the 10 or so equivalently expected uses of PI? \$\endgroup\$ – Daniel Zastoupil Sep 7 '18 at 14:53
  • \$\begingroup\$ @DanielZastoupil I never said they were equally common. Sunless Citidel (or more precisely its author) may not need passive Investigation. Lost Mines of Phandelver does have both from memory but not in equal numbers. \$\endgroup\$ – Dale M Sep 10 '18 at 4:32
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Passive Perception should be clear to anybody.

What I also use Passive Investigation for, is for example recalling things. Maybe you visited a house of a suspect, you later can, when new clues come up, recall details from there.

Think of like Monk (the TV series) when you think about it. He could maybe been hit on the head by someone sneaking unto him from behind without noticing, because I consider him having a low perception. But on the other hand he could maybe even recall the name of the books that were on the desk of the suspect when they talked to him.

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The rules on passive checks (PHB, p. 175) state:

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 GM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

Here’s how to determine a character’s total for a passive check:

10 + all modifiers that normally apply to the check

Passives are what you as a DM can use to describe who sees (or figures out) what. Instead of actively calling for rolls, which alert the players that there is something significant (or waste everyone's time if you're doing it just to be random) and also allow for failure to get across information you really want/need to convey.

One of the Acquisitions Inc. podcasts included the party showing up at an inn, and as Chris Perkins described the place, he said that Aeofel looked through the curtains and out a window to see a man out in the blizzard closing up a barn or some such thing. There was no roll called for, and while he likely was not using passives for anything in this case it is the sort of thing you'd use it for. The rest of the characters were doing whatever, and you can provide each with a little description of the place, painting a picture for the whole party and giving particularly observant characters a chance to notice something, which their player can then make use of in RP.

Passive investigation can be used the same way to provide hints and clues. Investigation is used actively to deduce clues that the character is considering, just as we'd expect. Passively, it can be the voice in the back of their mind putting things together almost subconsciously, as well as memories springing up upon coming across information. This again works when you want/need to get information to the party without alerting them that "something is here" and also injecting a very real chance of failure. It also keeps things moving along, because players could take forever to figure something out that their character would be far quicker at doing by virtue of being an actual investigative character and hero.

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