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So, I'm trying to figure out how to run the scene from the beginning of Lost Mine of Phandelver (5e starter set) where the party is ambushed by goblins and I'm really confused as to how Passive Wisdom (Perception) works.

The included rulebook mentions on page 6 :

When you hide, there’s a chance someone will notice you even if they aren’t searching. To determine whether such a creature notices you, the DM compares your Dexterity (Stealth) check with that creature’s passive Wisdom (Perception) score, which equals 10 + the creature’s Wisdom modifier, as well as any other bonuses or penalties. If the creature has advantage, add 5. For disadvantage, subtract 5. For example, if a 1st-level character (with a proficiency bonus of +2) has a Wisdom of 15 (a +2 modifier) and proficiency in Perception, he or she has a passive Wisdom (Perception) of 14.

So the party is being ambushed by goblins and at that point the DM guide mentions on page 7 :

Check to see who, if anyone, is surprised. [...] Make a Dexterity (Stealth) check for the goblins, rolling once for all of them. Roll a d20, add the goblins' Stealth skill modifier (+6) to the roll and compare the result to the characters' Passive Wisdom (Perception) scores.

So to me this reads like I should roll 1d20+6 for the goblins and compare that to the static Passive Wisdom (Perception) score on the character sheet of a PC. The PCs don't get to roll anything as they're not actively looking for enemies, even though they are investigating the dead horses.

I would also think that if the characters were supposed to roll, the DM guide would clearly instruct so, given that this is the starter set.

However, watching the video of WotC staff playing this scene out at around 8:01 I notice that the DM is explicitly asking his party for a perception roll. And the same thing happens on a couple of other videos where people are running the Lost Mine adventure. The DM always requests a roll.

So what gives? Shall I take it that the DMs ask for the roll because they consider the party to be actively seeking for hidden foes? Or am I just misreading the rulebook and the DM guide?

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Passive perception is exactly that, passive. It's what the PCs are always using when not actively searching for something and doesn't use a roll of the die.

To determine if you should use passive perception or allow a player to roll, listen to what they say their PCs are doing. If they say they are standing watch, keeping an eye out or something similar, they are actively searching so they can make a wisdom (perception) check, otherwise they are using their passive perception.

Although, it's kind of an experience call as the DM and depends on what the PCs are actually doing and whether or not the DM gives them the benefit of the doubt. In the case of the video, one guy said he was keeping a look out, thus actively searching and two of them were scouting off to the side of the road so the DM ruled they were also searching, rather than just wondering off into the bushes for no real reason, and allowed those three PCs an active wisdom (perception) check, whilst the others would be using their passive perception.

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    \$\begingroup\$ Although all answers were good and helped clear the confusion I'll vote for this one as the correct one because it also deals with the situation in the video. It does make sense in this situation for all the reasons you mentioned. Thanks everyone. \$\endgroup\$ – ctsag Sep 22 '14 at 14:19
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    \$\begingroup\$ Is it your house rule, or could you specify any PHB/DMG reference saying that passive/active check selection should be based on PCs doing something passively or actively? PHB page 175 has two points: "check can represent the average result for a task done repeatedly" and "can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice". Neither of that declares that DM should choose the check type based on how "passive" PCs actions are. \$\endgroup\$ – enkryptor Apr 7 '17 at 16:05
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    \$\begingroup\$ It's an extrapolation of the Hiding rules on PHB177. This approach is also used for trap detection on DMG120-121. \$\endgroup\$ – Gregory Avery-Weir Apr 7 '17 at 20:45
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    \$\begingroup\$ On listening to a Sage Advice podcast and rereading the rules, I think this is actually not quite right. Passive vs. Active refers to whether the player (or DM) makes an active dice check or uses the +10 instead. It doesn't refer to whether the characters (or other creatures) are actively looking or not. In fact, see the section on actions while exploring — only characters actively on guard use their passive perception. \$\endgroup\$ – mattdm Oct 10 '17 at 3:46
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    \$\begingroup\$ ... but the exploring situation is a special case. In general, passive perception acts like a floor, and in your example, the three PCs allowed active checks use that as an opportunity to beat their baseline passive perception. See this answer to a related quesetion for more. \$\endgroup\$ – mattdm Dec 14 '17 at 12:55
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This is all a DM's call, at this point.

In most of my games, unless the party is specifically looking around for trouble (having someone on watch while they pick over the horses for example), I would take the passive perception of everyone around. In fact, I might impose disadvantage to anyone that is currently checking something out and has their attention divided. This makes the most sense with the situation.

There are, however, DM's that like to have their players roll for everything, regardless of the passive Perception or not. This is more a preference thing about how the DM would like to run specific encounters

So, I'd say for you, unless the group is actively looking around for an ambush when they come up to it, take their passive perception. Otherwise, let the ones on watch role. It's just something you'll have to get the hang of.

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  • \$\begingroup\$ I don't think there's a way to apply disadvantage to a passive stat, is there? \$\endgroup\$ – Mooing Duck Sep 22 '14 at 19:28
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    \$\begingroup\$ Page 177 of the PHB in the sidebar about Hiding; Passive Perception: If the creature has advantage, add 5. For disadvantage, subtract 5. \$\endgroup\$ – Purple Monkey Sep 23 '14 at 6:21
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    \$\begingroup\$ I will add that my players at least always complain if they fail to spot something with passive perception. This is one of the many reasons I ask for perception checks. I also ask for perception checks at random times because I don't want them to know if I'm asking because it's important or because I want them paranoid. \$\endgroup\$ – Marshall Tigerus Nov 22 '16 at 21:47
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At 8:01 The DM actually explains this. Since the 3 stated that they are looking around at the woods while going to the cart the get to roll Perception. But the person who was walking straight there doesn't get the roll. Something else is going to happen.

BUT since the one who rolled who got a success and warned his party about the Goblins there was no need have a passive Perception Check.

If everyone who rolled failed the DC, than the person who was going straight through would have use their passive Perception.

Hopes that explains what is going on in the DM Mindset.

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Passive check is a DM's tool

Players don't decide when to make a passive check. It's the DM who might (but don't have to) use the passive score instead of asking a player for an ability check.

Passive checks in 5e are about players not making rolls (that's why they are "passive"), but DM still getting a check result. See the Player's Handbook, page 175:

A passive check is a special kind of abilily 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 DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

For instance, when an NPC tries to pickpocket a player's character, the player can make a roll, or DM can use a passive check instead. The rules don't strictly specify "use passive check here, and ask for a roll there", the decision is up to the DM, it's their area of expertise.

If the action result is evident, DM asks for a roll

DM uses passive checks to avoid awkward situations "you just made a bad roll, but I don't say you what that mean". Compare this:

— I'm trying to sneak past the guard, disguised as a peasant.
— Okay, make a Deception check.
— Ow, natural one.
— "Thief! Guards, get him!"

With this:

— I'm trying to sneak past the guard, disguised as a peasant.
— Okay, make a Deception check.
— Ow, natural one.
— ...
— So, what happens?
— Nothing. Go on.

In the latter example the player knows that something goes wrong, but has to pretend that he doesn't.

One can argue your passive score is your minimum check result

In certain situations you can't have a check result less that your passive one. This was confirmed in the April'17 Sage Advice by Jeremy Crawford. You should hear the whole podcast, but basically he said that your Passive Perception is always "switched on", unless you are unconscious.

He also said this can be applied to all ability checks, not only to Wisdom (Perception). However, this overlaps with the Automatic Success variant rule (DMG, page 239). I suppose that means that when the situation is peaceful and nothing threatens the characters a player can choose passive score or use a roll result, whichever is bigger (like "Take 10" from the previous D&D version).

DM still can ask for a check and use its result, even when the passive score was bigger, if she/he thinks this is appropriate. For a Wisdom (Perception) check a low roll could mean you was distracted in the middle of the battle and didn't notice something obvious.

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Passive checks in general can be used whenever the DM wants to avoid dice rolls, either to speed things up (using the passive check as the average result over a period of time) or if the DM wants to determine a result secretly.

There are some special and specific rules for Passive Perception related to exploration and hiding. In the Movement section of chapter 8 of the PHB (Adventuring), there's this bit:

Noticing Threats

Use the passive Wisdom (Perception) scores of the characters to determine whether anyone in the group notices a hidden threat. [...]

Characters who turn their attention to other tasks as the group travels are not focused on watching for danger. These characters don’t contribute their passive Wisdom (Perception) scores to the group’s chance of noticing hidden threats.

This rule actually shows that despite the name "passive", it doesn't mean that this applies only when the character is passive ­— in fact, characters must be on the alert for danger to even get a check. It's just called "passive" because there is no active roll.

Jeremy Crawford goes into some detail explaining this in the Sage Advice section of this podcast, starting at about 15:09. (See this related answer for a painstakingly-hand-created transcript.)

In short, use the Passive Perception of all observers to set the target when you want to hide on your turn. (You have to beat their number, though — remember that when contests result in a tie, the situation remains unchanged.) If you beat all observers, you are hidden. Then, if other side wants to search for you on their turns, they can make active Wisdom (Perception) checks against your Stealth total (again, needing to beat the number to change the situation, which is now that they don't know where you are).

Jeremy Crawford also notes that Passive Perception is active at all times, and at 23:20 he explains that Passive Perception is always on, and represents a "floor" to your perception. He says that if you get an active roll lower than your Passive Perception, Passive Perception should be considered a minimum. But he also says that if DMs would remember to use Passive Perception as intended, you'd already know about anything with DC lower than your passive perception without a roll, and there'd be no call for an active roll in the first place.

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Here's how I run it, and I'd guess this is how they run it as well:

if your players meet the passive perception DC set by the goblins' stealth rolls, they notice them right away. But if they approach cautiously (as many D&D players are wont to do), and they are checking around the horse and keeping an eye towards the tree line for a likely ambush, then they'd get an active roll to try again.

This is how passive perception worked in 4e, and it's how it appears to work in 5e (though in 5e DCs are often explicitly higher for passive scores).

That said, why they do not give guidance for this in the starter set is definitely a question. I think there's a good reason for it. It's much simpler to just take the passive scores and trigger the ambush. This is supposed to teach both the players and the PCs to be careful. If they are already being careful, whether from previous D&D experience or simply from an abundance of caution, it makes sense to reward that with the opportunity for an active roll (I'll make no qualitative statement on whether this is a good or bad thing, it's a table level decision).

As this is the very first encounter in the book, it makes sense to delay discussing active perception and just introduce passive in the first chapter. If you want to introduce active in your game sooner, go right ahead.

So when you run it. You now know 2 different ways to handle this situation. You can play it by the book and let your party eat the surprise round if the goblins manage to hide and your PCs aren't paying attention OR you can reward them if they are cautious with another opportunity to roll the check and maybe spot the goblins. It's entirely up to you.

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Passive Perception is essentially a convenience, for both the DM and the players, preventing an over-abundance of rolls at the table from crippling the pace of the game. Remember that fact first and foremost when deciding whether or not to use it.

In years gone by it has not been unheard of at my table, especially in known "killer" modules like Tomb of Horrors, for players to have their PC's check every five feet of corridor for traps, hidden switches, and the like. Passive Perception (and in 4E, Insight) were designed to alleviate this problem, giving players a reasonable chance to see stuff just because it was there. Some players like this, some don't. Many players just love rolling dice and dislike anything which stops them doing that.

In the sessions of 5E I've played, the Passive score has been king. PC's earn the right to make a Perception roll by calling out their actions, but it doesn't replace the awareness granted to them by their passive score.

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You can do it either way, but by rolling for the goblins in secret, you keep the goblins' presence secret to the players. Having the players roll for perception tells them that there's something to be found, and it's easy to take that as an indication that they should start searching. When you compare their passive scores to your roll, you avoid giving them knowledge their characters shouldn't have, which might bias their future decisions.

It won't make any mechanical difference either way, it's just an option if you want to avoid out-of-character knowledge influencing the game.

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Passive skills are how the DM determines how the PCs can observe, like with Passive Perception, and do other things without actively intending to. Basically, the DM calculates the score by adding all regularly applied bonuses to a score of 10, the average roll on a d20. The DM would then calculate the Passive Perception of the PCs for their observation of the Goblins, as they are not actively searching. Rolls are for active uses of skills, while Passive is the average trained success rate. Also, like what has already been said, a roll would alert the players of the existence of something to search for. If a player actually asks to make a Perception Check though, use an Active one.

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