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This actually a number of questions that conform to the title question, and which all stem from a bit of rules confusion.

Let's start with the rules confusion.

On pg 178, under the skill Investigation the rules says the following;

You might deduce the location of a hidden object,...

On the same page, directly to the right of the investigation skill, is a green box titled Finding a hidden object. In the box the rules says the following;

When your character searches for an hidden object such as a secret door or trap, the DM typically ask you to make a Wisdom (Perception) check.

Now here we have two skills that clearly does the same thing.

So my first question is: Are the skills interchangeable or mutually exclusive?

That is: can you use either skill to search for traps in any situation, or do you use Investigation only in specific situations you can't use Perception in, and Perception only in specific situations you can't use Investigation in?

My second question is: What do you use to make passive checks to spot a trap?

Do you only use Perception, as you would if a character did not have proficiency in either skill? Or, if a character is proficient in Investigation but not Perception, would you then use Investigation instead?

Which leads me to my last question: What if a character is alert to the possibility of hidden objects like traps, but isn't actively searching for them?

In other words, what happens if a player utters the oh so familiar words: "I'm prepared for the possibility that there might be traps in here." He/she isn't actively searching, but it isn't exactly passive either. If the character has proficiency in Investigation, do you then use that skill, or do you stay with Perception checks?

In other words: when do you use what skill?

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Perception is for observation, Investigation is for deduction.

Some of this answer will be observations on how Wizards has done it so far and some of this will be logic, and some of it will be mechanics.

First for the headline question. It depends. when to use Investigation, and when to use Perception is not entirely clear yet, and I'm hoping we'll get more guidance in the DMG late this year. The guidance from the rules is that the two skills mostly seem to differ in the methods by which they are found.

Depending on the exercise, either, or both of the skills may be used.

For passive checks, you're almost always looking at Perception being the skill of record. While all skills can be used passively, some skills make more sense than others. Perception is the hallmark passive skill, whereas investigation makes less sense as a passive skill.

If the character is alert to the possibility of hidden objects/traps, but not actively searching, he's using passive perception.

The guidance for this seems to be (though we can't confirm yet), that the DC for actively looking for something is regularly about 5 less than the DC for passively looking for something (or sometimes actively looking always reveals it).

So here's how I would play it. Traps can be noticed with perception passively (usually DC 15). If the PC is looking, ask them how they are looking. If it's observational, then use Perception. If it's deductive, use Investigation. When they are searching for items, again, either skill is appropriate. This is somewhat counter to how WOTC has written adventures so far. They are always written to use perception to notice traps. Passive with a higher DC and active with a lower.

Investigation also has broader uses such as when you are trying to track clues, or put something together. It's also a great "roll for a hint" kind of skill if your PCs get stuck and need some help figuring out what to do next.

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  • \$\begingroup\$ I had a player character throw away a key in a dark cave. They then realized they needed it and went to look for it. I thought investigation... do you think it depends on how they look though? \$\endgroup\$ – Jeff Mar 11 '16 at 17:16
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    \$\begingroup\$ @Jeff Yes. And depending on how involved I wanted to get with a situation like that, I might well use a combination of investigation (to figure out where to look) and perception (to actually look). That'd also be a situation where fail forward is very much in play. You definitely find the key, but depending on what you roll determines how long it takes. \$\endgroup\$ – wax eagle Mar 11 '16 at 17:38
  • \$\begingroup\$ Written before sight of the DMG, this answe turns out to be a bit fuzzy. \$\endgroup\$ – GreyWulf Jul 5 '18 at 20:05
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Passive Perception is always on

When characters are just moving around, sitting around, doing other things, they still always have their passive perception. They can spot traps and detect enemies hiding whenever they would come within sight/hearing range of the object/creature in question. As a rule of thumb a DM should have all of the PC's passives already noted, because asking for someone's passive is tantamount to saying "ITS A TRAP" and will instantly cause player to start making active perception checks.

When searching for traps & possible enemies, use a Perception Check

Finding a Hidden Object

When your character searches for a hidden object such as a secret door or a trap, the DM typically asks you to make a Wisdom (Perception) check. Such a check can be used to find hidden details or other information and clues that you might otherwise overlook. In most cases, you need to describe where you are looking in order for the DM to determine your chance of success. For example, a key is hidden beneath a set of folded clothes in the top drawer of a bureau. If you tell the DM that you pace around the room, looking at the walls and furniture for clues, you have no chance of finding the key, regardless of your Wisdom (Perception) check result. You would have to specify that you were opening the drawers or searching the bureau in order to have any chance of success.- PHB p. 178.

Instead of taking the passive score, the players should be rolling perception checks while they search the room/area.

Investigation exists in a murky area between Perception and Insight.

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 o f weapon dealt it, or determine the w eakest 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. - PHB p. 178.

Whenever your PCs are trying to do their best impression of Batman or that dude from CSI with the glasses, or the cops on law and order, this is what they would use.

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Xanathar's Guide to Everything expands upon traps and how to find them. In the examples they give, virtually all of them use a Wisdom (Perception) check to detect the presence of the trap, and a Dexterity (Thieves Tools) check to disable or bypass it. Some magical traps instead used Intelligence (Arcana) or Wisdom (Religion) to disable it. Investigation was not even mentioned on the section about traps.

It is, however, mentioned in the section on Tool proficiencies, where they say what skills might benefit from proficiency with Thieves Tools:

"Investigation and Perception. You gain additional insight when looking for traps, because you have learned a variety of common signs that betray their presence."

So it would seem that this is still a case-by-case basis and largely up to the DM.

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The simple rule of thumb is that Intelligence(Investigation) is used when the character is actively searching for something specific (even if it's a generic sort of "clues" or "traps).

Wisdom(Perception) is used when a character is passively able to have a chance to accidentally discover something, or is on the lookout to possibly find 'something' then the players roll perception or the DM uses passive perception.

For my own games, I require that the environment is "interacted with" when doing an investigation, and I require that the environment is "not interacted with" when doing a perception check.

For example. A player says they wish to poke at the stones to see if any are a preasure plate, or have hidden things in them. Then I tell them to roll investigation. However, if the player says "I look and listen around the room to see if anything is amiss", then I ask for a perception check.

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From page 178 of the PHB:

Investigation. When you look around for clues and make deductions based on these clues, you make an Intelligence (Investigation) check.

Perception. Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something.

In other words, a player could use Investigation to find a trap having detected a clue (e.g. loose dirt or oddly shaped tiles), but to see the clues (or the trap) in the first place, Perception is the check required. If the clues are presented by the DM as part of the description, the character automatically knows them and can use Investigation without requiring Perception.

For published examples, there are a number of traps in HotDQ and the Starter Set. All of them require Perception checks, not Investigation. In fact, none of the published traps mention Investigation at all. I would personally allow it if they notice something suggestive of a trap, but (as per the question) Perception definitely seems to be the only option.

To address your 3 specific questions:

  1. Are the skills interchangeable or mutual exclusive? Definitely not. They are distinct skills with different uses.

  2. What do you use to make passive checks to spot a trap? See above, you use Perception.

  3. What if a character is alert to the possibility of hidden objects like traps but isn't actively searching for them? This is tricky, since the recognised states are effectively 'searching' and 'not searching'. I would suggest that heroic adventurers are always 'alert', so this is the same as not actively searching.

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    \$\begingroup\$ Still that's kind of harsh having a character succeed on a skill check to be able to use another skill. \$\endgroup\$ – Chryckan Sep 4 '14 at 12:53
  • \$\begingroup\$ @Chryckan actually it's pretty reasonable, see 4e's skill challenge mechanics. More rolls is actually a good thing with skills. Further reading: ponderingsongames.com/2013/01/27/goblin-dice \$\endgroup\$ – wax eagle Sep 4 '14 at 12:55
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    \$\begingroup\$ Yes, it's the same in the starter. All traps are perception. As a DM I'd permit Investigation with a strong argument (specially if clues are dropped when the scene is described), but it's pretty much always written for perception. \$\endgroup\$ – wax eagle Sep 4 '14 at 13:02
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My thoughts are that there are (3) things going on:

1) Passive perception. This is used primarily as an opposed check that the player does not know he is making (e.g. vs. stealth; or to "notice" a thing (like a trap) without actively looking). For this example, let's say it's a trap on a rope bridge that is DC20 to notice.

2) Active perception. The players before proceeding across the rickety bridge want to actively check it for traps, feeling that even though they didn't "notice" anything - there's a strong possibility of a trap here. In this case, they are rolling. In this example, I assign a DC15 to find the trap. Furthermore, I might give them a lower DC (maybe, 10) on the same roll, to not detect the trap outright, but to notice something amiss: "You think the ropes look off, somehow. Thin, perhaps."

3) Investigation. If they failed to find the trap, but notice the funky ropes, this is where a character with investigation can say "Aha! Let me investigate the ropes and the bridge further, to see if there's anything going on here." This would be a separate roll, probably with roughly the same DC as perception. It gives the players a last-chance effort to detect the thing and save their bacon.

Another idea I have heard, though I haven't played with it yet, is that perception is the only detection skill, but a rogue can use investigation results to lead to advantage or disadvantage on the disarm.

Finally, you can take this to extreme, and NEVER let them outright detect the trap with active or passive perception, but rather always make perception give only the starting hint, and require investigation to then deduce the trap. I'd be wary of this approach, though, because then it actually requires 2 sequential successes on 2 rolls, as well requires a character versed in investigation; rather than the above approach which gives players a "failure" path that leads to success.

Of course, all of the above approaches have a place (and are not mutually exclusive within a single session), and the beauty of 5th edition is "simplified rules leads to less rules lawyering, more DM adjudication, and more fun". The drawback is never knowing when you join a new group exactly how the game works, but I favor the former over the latter, personally.

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Perception is used for senses

"You walk down the hallway and you see a row of tiny holes along the bottom of the right-hand-side wall, what do you do?"

Primary sensations you can see or hear or smell or touch or feel in the air or similar. PHB p 178.

Investigation is for knowledge & reason

"Yes, the lack of dust in a quarter circle in front of the wall does look like a secret door has been recently opened here, and then closed again."

Knowledge of architecture, design, common sense… and reason to draw conclusions from the clues you've used perception to gather. PHB p 178.

Active vs Passive

When you're just moving in the dungeon with your eyes, ears or nose open, sometimes you (as a character) perceive things even without you (as that character's player) asking to actually look. As an example, sometimes the character sees a tripwire even without the player going "I'm going to look to see if there's a tripwire" in every room.

Using a skill passively means walking around with a permanently rolled 10 (or 15 if you have advantage), modified by your bonus, and then compared to the various DCs. For example, a particular tripwire might have a passive perception DC of 15 and then anyone with passive perception 15 or higher will see it without actively looking. A set of footprints might look weirdly overrepresented by steps leaving a small, closed room, and anyone with a passive investigation of 15 or higher will realize that without actively pondering it.

Suggestion

Passive perception is OK. It allows you to selectively describe the dungeon, normally to everyone and then to high-perception characters add things like "[character name], you see a tiny crack along the left-hand-side wall". Some modules have DCs already listed. When writing homebrew locations, or prepping modules that don't have DCs listed, write the DC down beforehand and stick to it. One idea is to roll 2d6+8 (or 4d6+1, or 4d3+7) to get a DC that's centered around 15 [the standard, "medium" difficulty] but doesn't make any single perception score actively best (i.e., no "the optimal passive perception to shoot for is 15, it's worthless to go lower and a waste to go higher"). Again, do that roll beforehand so you can write it down & stick to it.

Active perception, and both passive and active investigation, interacts weirdly with player skill. Therefore, don't use them. (It's true to some extent to passive perception as well.)

"I open the drawer and lift up the clothes, do I see the key there?"; this is Perception. PHB p 178 has a similar example.
But if the prep says that the figurine is actually in there, should you then really have to roll to see if you can see it? At some point it becomes ridiculous. "I grab my sword!" "OK, roll to see if you can feel your own sword."

The PHB's example points out that if the key is in the drawer under the clothes, looking at the walls of the room would give you no chance of success. So obviously player skill can enter into the equation, and you have to decide how player skill and character skill should interact.

So you have four options:

  • The character skill becomes a lifeline for when player skill is insufficient. You let them find the key without them looking under those clothes, and if they don't look in the right place you let a high perception roll give them another chance to find it. (This seems to be the implied option from the PHB text; their skill leads them to the drawer, the roll leads them under the clothes.)
  • The character skill becomes an additional hurdle to jump even when player skill is high. You don't let them find the key even if they look in the exact right place until they've also made a perception check.
  • The player skill is ignored. Don't dwell on precise description until you've seen the outcome of the die rolls. It becomes "Roll to solve puzzle."
  • The character skill is ignored. Don't roll these skills and instead rely solely on player skill.

Decide (together with the group) which of these four interactions you prefer and apply it consistently.

As a side suggestion, if you do go without using a lot of skill rolls—for example, if you are also not using Persuasion, Deception, Intimidation and Insight, for the same reason—is the Ability Check Proficiency variant in the DMG on p 263.

As an example, our group uses the fourth option and that ability check variant. Last week, we played two four-hour sessions and in those eight hours of gameplay there was a total of four ability checks. (One strength check to grapple, two strength tests to budge open doors, and one int check to recognize that a book about golems was specifically only about clay golems.) It's perfectly fine to play D&D without every action or statement being accompanied by an ability check.

Pouring out flour to see trip wires, pouring water to see trap doors.
Just describe the room and ask "what do you do?"

What's fun to roleplay out vs what's fun to roll is different between groups. You might want to roll for trap finding if you hate pixel hunting but then play out social interactions, or you might want to play out trap searching if you love exploration but then just roll to quickly resolve social interactions if that's what you want to do.

Exceptions

When is that fourth option no good?

  • When you are winging it and you don't know if the key is in the drawer or not because you don't have that in the prep.
  • When the group has no desire for player skill but instead are highly invested in finding out "what would the character have done in the situation?"
  • When there's a large time skip: "OK, we search through every room on the top first two floors. Three days later, have we found anything?"
  • When you desire a high differentiation between characters; a low-int character being played ignorantly and a low-wis character being played obliviously.
  • When you desire a hilarious amount of slapstick and mistakes. "Oh, wow, I can't believe my character didn't see that tripwire even after I looked so carefully, hahaha, well, time to roll up a druid, I guess."

If these exceptions apply to you, so that you still want to use these skills, hopefully the differentiation at the top of the answer can be useful.

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