Hey,
the following thread discussed the question, when to use investigation and perception checks on object finding or traps. As an result of this there came out several points for DMs and players about how to handle situations including these checks. Some of them are RAW , some are advices given by "veteran" players. Special thanks to: iserith, Codedump00, paulstrait and Psikerlord that explained everything with a lot of patience! 
The following is a summary of this thread:
If the charater spotted something suspicious (maybe via his Perception check) he can try to interact without making an investigation check (like taking a telescoping pole and poke around the cover of a hidden pit). The DM can ask for an investigation check, if the action, the player describes has an uncertain outcome or the player doesn’t know what to do or where to start. As an example this could be because of the complexity of a trap or because the player simply doesn’t know how to avoid it and provokes a check himself (e.g. "Chuck Dagger was looking for hidden threats and these tracks seem suspicious. He tries to deduce what this clue means."). Again, even then there is not always a need for a check, because maybe the DM thinks that the character might know such things, but the player simply doesn’t get it. Only if the DM thinks it is uncertain, the player will have to roll an Investigation check. There also may be times where one aspect of the Perception/investigate or find/understand process will be automatic. Actions to avoid the trap from being activated may be possible without a check. Therefor the better the DM telegraphs the situation and the environment, the better the player can react even if he failed a check (and maybe won’t be able to spot every detail of the trap or disarm it). Finally in most cases players will benefit from detailed descriptions of what they are trying to do. E.g. a character investigating a trap by poking around with a pole trying to uncover a pit could possibly be awareded by getting advantage for further investigation checks (if the DM asks for a check and decides so). But in some cases there also might be the risk of failure (or disadvantage).
Concerning traps:
For example: A tripwire may be difficult to see and require a DC perception check by default to find. But once found it is trivial for any PC to follow the wire to the jar filled with poison. This 'investigation' could be an automatic success to understand the trap and how to disarm it.
Similarly, another trap may be obvious (e.g. you can see the murder holes or the the metal blades or the openings for the acid to spray out), but it will require a high DC for investigation by default to understand how it works or how to disarm it.
A Wisdom (Perception) check, passive or otherwise, may allow a PC to notice a hidden clue.
An Intelligence (Investigation) check, passive or otherwise, may allow a PC to discern what that clue means.
Of course, remember that the player must describe an action for his or her character and you as DM must determine whether or not the action as described will succeed, fail, or has an uncertain outcome. It's only in the latter case that an ability check is called for.
Consider describing the environment in an interesting way that invites the players to engage with the exploration pillar by telegraphing threats and clues. In the doing, you will encourage players to make deductions about their environment and take actions accordingly. They can therefore sometimes rob randomness of its power and solve for X without ever needing to roll a die.
While its not an exact process... I view perception as noticing something, and investigation as discovering something.
EX. If you want to find a secret door in the south wall, that would be Investigation.
Noticing that one wall sconce was at a slight angle would be perceptoin.
If you noticed the wall sconce, I might give advantage on the Investigation.
Or perhaps you would have disadvantage on the Investigate unless you specifically mentioned the wall sconces.....
I often the two interchangably, asking for a wisdom perception or intelligence investigation check , player's choice. I've not noticed any downside to this approach.
My system is perception covers finding everything, investigation covers finding static things like clues, secret doors, hidden items. Investigation gets two benefits one I drop its DCs by 5 and two it covers some levels of interpretation of the found object.
Thanks for your answers!
It still find it quite hard to distinguish between those two ability checks, because they seem so close.
iserith wrote:
lkwpeter wrote:
Thank you for your thorough answer, iserith!
Now concerning finding & disarming traps:
As I read through the DMG it seems like several traps need both Perception and Investigation to a) recognize the trap (Perception) and b) find or deduce the trigger (Investigation). I first thought you would need either one of it or the other, but it really seems you need both. Furthermore you need to make a third check on Thieves' Tools to disarm it. All I can say is: Wow! In 3.5E this was easier!
Would it be possible to retry the search for a trigger several times (DMG pg. 237) or succeed even without a check (DMG pg. 121) , if I describe detailly what I do? For example: "I am searching the floor very carefully, gently pushing the floor tiles to find a possible trigger."
How do you think about it?
Kind regards,
Peter
lkwpeter wrote:
I hate when a post gets eaten, and I have to retype it.....
lkwpeter wrote:
Oh man, thanks a lot for your great description and all the work you both put in it!!! :-)
I think I got the main point:
- The better the player telegraphs his action, the less a succeed will be decided by a roll (both Perception and Investigation).
- Furthermore a check is only relevant if it is uncertain if the action, the player describes has success (either because of the circumstances or the lack of description the Player offers) Therefor certainty seems to be the most important word of the description in your posts and the PHB, they should bold it).
Is it right that perception still is more important to be good at than investigation, as you can avoid investigation checks far more than perception checks (either you know the general mechanisms or you simply avoid the trap)?
Last question: If I spot a trap, but won't be able to investigate how it works. Will I be able to avoid it like walking around or step over it, although the missed investigation check? I already can hear my DM say "If you don't manage to investigate the trap, you are not able to *find* the pressure plate. So you can't avoid it actively. Just hope that you are lucky."
Thanks again and kind regards,
Peter
lkwpeter wrote:
lkwpeter wrote:
lkwpeter wrote:
lkwpeter wrote:
IMG:
Perception is for non-interactive discovery (ie - looking, listening). You can use perception to overhear rumors at a tavern, notice the askew sconce, spot a trap before you blunder into it, and detect an ambush before people jump out from behind rocks and yell surprise.
Investigation is for interactive discovery (poking, prodding, holding up caterpillars in jars). You can use investigation to interview tavern patrons, prod your 10 foot pole around to find a trap, knock on the walls to find secret doors. It can't help you against an ambush.
In general, perception is something I as the DM roll in reaction to the environment, while investigation is something the player asks me to roll.
Ive decided I am going to try merging investigation into perception. Any rules reference to Investigation is substituted for Perception. Choosing either Wis or Int, player's choice, each time a roll is needed. Avoid a lot of uncertainty. Probably little downside. I think.
Hey all!
Thanks again for all your posts. This thread has almost become a "mini guide for spotting, investigating and traps". Because of this I wanted to summarise the most important facts and put it in the first post a(of course marking it as an an exerpt from your help). Could you please check the following states, if there are correct and if I might have missed something. Remember I wanted to keep it short. :-)
Summary wrote:
Just a couple of possible tweaks:
Point 2: That is less a rules interpretation than a DM playstyle advice, not sure if it fits with the rest of the items. Some DMs like to telegraph, some like to leave the situation 'natural'.... (Also check Point 11)
Points 5&6: Players don't ask for ability checks, they just declare what they are doing, the DM decides if an ability check is needed. The player may inspect the item, or use a pole to push down on the 'trap' or whatever.... the DM decides if this works, or if this calls for an investigation check. Even if the PC says "I want to investigate the box" that is declaring an action, not making a check. You cover this in Point 7, but the terminology in 5&6 don't match that.
Points 9&10 I would remove the "(normally)", since the meaning doesn't come across. (Trip wires are not 'normall' DC 15; which is what is sounds like) Instead, I would use Point 12 as Point 11, and have it include both investigating and perceiving.
Summary wrote:
Any ability check is only made, if it has an uncertain outcome. So it’s not necessary to call for checks, if the player is performing a task that will clearly succeed or fail. This guarantees that the game doesn’t get stuck or being left on randomness of die rolls (for trivial tasks).
The DM should telegraph hidden threats and situations that could include more possibilities to interact with someone or something to the player by describing the environment in an interesting way, so that they have a chance to react to that situation.
Perception is to notice a hidden clue or find something in the evironment (e.g. an object) by spotting, hearing or detecting its presence.
Investigation is for interpreting hidden clues, so you understand what it means. It does not deal with detecting the presence of clues per se - it deals with deducing things from the clues you have in hand if said deduction is not a certainty. Oftentimes a player will simply draw conclusions on his or her own and act accordingly, thereby skipping the need for an Investigation check.
An investigation check can be initiated by the player though by trying to proof the validity the deduction (= players theory). E.g. "Chuck Dagger considers what these clues mean and tries to deduce whether [player's theory] is correct..."
iserith wrote: