When is each skill used when searching for hidden objects like traps?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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 rules-as-written (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, rules-as-written 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.

Answer 6

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.