In the beginning, when a party of adventurers encountered a group of monsters, both sides rolled a six-sided die. If a side rolled a 1 or 2, that side was surprised and had to stand by while the opposing side got a free round of attacks (or of fleeing, if the opposition looked scary).
As the game developed, certain characters and monsters changed the chances of surprising or being surprised. Rangers made it less likely the party would be surprised. Bugbears surprised more often. The Advanced Dungeons & Dragons rules became a marvel of incomprehensibility when it came to the surprise rules, with a wealth of competing and incompatible subsystems. Everyone sighed in relief when the 2nd Edition rules were released, and the surprise rules all used the same system. It still was “roll a dice, if you roll low you are surprised”, however.
Things changed with Dungeons & Dragons 3rd Edition. This set of rules had a comprehensive skill system, with Listen and Spot skills that allowed a character to detect whether a monster was Moving Silently or Hiding. The “both sides roll a surprise die” rule was gone. Instead, it was the interaction between skills that determined whether a battle began with a surprise situation. In addition, the role of the Dungeon Master in determining whether surprise was even possible was made stronger; many situations had neither group even attempting stealth, and thus surprise would not occur.
One interesting point about 3rd Edition: These checks were opposed checks. Thus, a 1d20 + skill modifier vs. 1d20 + skill modifier. This will become relevant shortly.
Fourth Edition introduced Passive Perception. Instead of requiring the player to make a check whenever there was something hidden around, it was assumed the player rolled a 10. This rule has carried on into the current edition of Dungeons & Dragons. Each character has a Passive Perception score, equal to 10 + their Wisdom (Perception) modifier. Whenever something is around that the characters might notice, the DM can compare its Dexterity (Stealth) check to the characters’ Passive Perception scores, and quickly determine if they notice it.
This is a simple system.
The system is at its best when dealing with creatures. In this case, it is incumbent for the creature attempting to be unseen to make the roll. The fact that there’s a roll involved brings in an element of chance and risk. Good D&D play tends to not like “sure things”.
The reason that the rolling goes into the DMs hands is so the players aren’t alerted to when something important is coming up. Getting the players to make Wisdom (Perception) checks when they’re walking over an ordinary floor tends to give the game away.
It has the advantage of not being an opposed check. Why is this important? It’s because of probability. For each +1 in the check bonus, there’s a flat 5% increase in the possibility of success. When you’re dealing with opposed checks, the chances change in a non-obvious manner, with even a small differential causing a great advantage to the more skilled participant. When most of the game uses flat rolls, having an opposed roll is very noticeable: the check doesn’t work the way you expect.
Because the Dexterity (Stealth) and Wisdom (Perception) checks use the same system to derive their bonuses, and thus the Passive Perception scores, they’re comparable: both bonuses exist within the same range. (This was a major flaw with D&D 3rd Edition, where skill checks weren’t even comparable. If you remember the system, consider all the synergy bonuses Diplomacy could get!)
It also makes determining whether a Dexterity (Stealth) check succeeds very easy – you don’t need to get everyone to roll to see if it succeeded. Just compare against Passive Perception scores. Most DMs make a note of their players’ scores at the beginning of each session.
Of course, a player can always use an action to make a Wisdom (Perception) check when they need to see a hidden creature!
Traps give the system a few problems.
One of the problems here is that it’s not clear what the difference between Wisdom (Perception) and Intelligence (Investigation) is. Some traps require Wisdom (Perception) and others require Intelligence (Investigation)? Even the rulebook flounders around on this one and doesn’t give a clear answer; and the adventures tend to make Wisdom (Perception) king and make Intelligence (Investigation) a poor cousin. In my games, I tend to go with Intelligence (Investigation) as the main trap-finding skill.
Another problem – and this doesn’t worry me that much, but does some of my friends – is that traps you can spot with Passive Perception become irrelevant unless you set the DC at a very high level. And there’s no die roll involved. Now, I like a party to be able to find traps if they’ve built characters who are good at finding them. It’s a reward for their effort. It also pays to have traps that are significant even when found. A pit in the middle of a room which can be walked around isn’t that interesting; a pit in a 10-foot-wide corridor blocking passage even when found is interesting, as the players now must negotiate it. The “trick” here is to think more about traps.
The more significant problem is that there’s no dice roll involved. The trap has a detection DC. The character has a Passive Perception score. Compare one to the other. It’s dull and boring, and entirely too predictable. Designers set DCs at levels that are stupidly high, just because they’re sick of the players finding traps.
The problems with Passive Perception are made even worse by two feats: Alert and Observant.
Alert says a character can’t be surprised. However, it doesn’t say the character knows where the monsters are. This gives rise to the following situation: “Roll initiative!” “I win!” “Why did we roll initiative?” “You don’t know.” “I don’t know what to do!” I don’t like feats that lead to this sort of situation – and I hate even more feats that just flat-out stop a situation from happening.
Observant gives a +5 bonus to Passive Perception (and Passive Investigation, whatever the latter means…). This is a massive bonus, which just breaks the mathematics of the game. It’s game design I really dislike. The design process probably went like this: “We’ll give the Observant character advantage on their Wisdom (Perception) checks, but only the passive ones. Oh, advantage doesn’t mean roll two dice when applied to a passive check, it’s a +5 bonus instead. We’ll just say it’s a +5 and leave out any mention of advantage.” This leads to situations where a characters Passive Perception score is in the low twenties or even higher. It’s not too bad against monsters, as they still have a chance of hiding, but against the flat DC of traps? Not so good.
It’s not entirely defined what happens when a player uses an action to spot a hidden creature (and makes the check). They’ve used their action. Can they do anything else? Can they point it out to other players?
If I could have my way, I’d likely ban a few feats from the game, Alert and Observant being two of those (along with Sharpshooter and any feat that removes the penalties of missile weapons, but that’s another article!). This would normalise the mathematics, and remove strange exceptions and the situations they create.
The most significant variation I’d add would be to change how detecting traps worked. Instead of using a flat DC, I’d incorporate a roll into it. Thus, a trap would have a detection DC of 1d20 + 6, or +10, or +12, depending on how hard it was to detect. That way, it introduces an element of chance, allowing characters with good Passive Perception scores to still detect traps more often, but removing the certainty caused by having a good Perception DC.
Yes, you could put the roll in the hands of the players, but the idea of keeping danger hidden from the players unless they detect it is a strong one. If you disagree, throw out Passive Perception and go back to requesting Wisdom (Perception) checks when the players need to notice something.
My house rule for what happens when a player perceives a hidden creature using an action is that it gives advantage (and thus a +5 bonus to Passive Perception) to all other checks of characters. This might allow them to automatically spot the hidden creature. At the very least, the square it occupies is identified and can be attacked.
Those are a few of my thoughts on Passive Perception. To listen to the podcast that inspired this article, go visit Down with D&D. It’s a fascinating podcast, well worth listening to!