Ambush: one encounter - two outcomes

[Hilary]

So, I am imagining an ambush encounter where five bandits have dug ten foxholes overlooking a road. They have a trenches that allow them to move from foxhole to foxhole and pop up and attack from being hidden. With surprise and some crossbows, a group of 1st level PCs could easily lose one or more members of their group.

Yet, if the wizard has an owl or hawk familiar scouting ahead, the bandits are easily detected and the group of PCs can avoid the ambush and even get the drop on the bandits by approach them from their blind spot.

One encounter - two vastly different outcomes. CR rating is the same for each.

[meandean]

Sure. CR is a self-acknowledged very simplified rating. The unspoken implication is that, once you're past the "tutorial" stage of DMing, you shouldn't need it anymore.

[Lunali]

CR is a rough gauge of the amount of damage a creature can dish out and take while engaged. No more, no less.

Anything more complicated than "you hit it, it hits you" falls into the territory of the DM to figure out the balance.

[PhoenixPhyre]

CR, as others said, is a creature statistic, not an encounter one. And it explicitly, intentionally does not consider tactics, synergy, or environment. That's not what it's designed to tell you. It's a first-run filter for DMs, under a certain set of assumptions. And that's all it claims to be, if you read the DMG.

No system can account for all the variables, unless you're in a board or computer game where the possibilities are super constrained.

[Cheesegear]

One encounter - two vastly different outcomes. CR rating is the same for each.

Classic misconception.

CR does not mean 'difficulty level'. CR means, in very simple terms, how much damage a creature can deal, and how much it can take (generally, over three rounds...After that, who knows what's happening?). That's it.

This becomes blatantly obvious when you start creating your own monsters; What does, and doesn't, increase or decrease CR, is directly related to the creature's ability to deal and mitigate damage.

What isn't factored into a monster's CR, are terrain variables and environmental effects that increase or decrease their ability to deal or mitigate damage outside of what's written in their statblocks.

For example, a hostile creature inside a foxhole, with a ranged weapon, can pop up, deal damage, and use its Move action to dart back into the hole, gaining Total Cover. A creature with total cover can't be targeted, and can't be affected by several abilities. That means that a monster can deal damage, and take no damage at all, in return. Taking no damage at all and being immune to several abilities and targeted spells, is literally one of the best defenses any creature can have...But if those defenses come from the environment, and are not part of its statblock, then its CR wont change.

A very experienced DM might know that if 'Environment Does [X]', then that hostile effectively has that offensive or defensive capability in its statblock, and alters its CR accordingly.

But, as the OP very rightly points out, environmental effects can very often be negated by a prepared party - sometimes without even using a resource. But that depends on the players. You can always add more hostiles into an encounter, either when the fight starts, or just add in a phase two to the fight if they dominate it quickly.

[Hytheter]

CR is a rough gauge of the amount of damage a creature can dish out and take while engaged. No more, no less.

Anything more complicated than "you hit it, it hits you" falls into the territory of the DM to figure out the balance.

They oughta put this on a t-shirt.

[Pondincherry]

The difficulty of an encounter STARTS with the CR of the monsters versus the party's character level, but then you have to go in and modify it due to any advantage one side has over the other. There's suggestions for how to do this in the DMG, but basically you'd realize a Hard encounter on paper is actually a Medium encounter if the the party has a familiar scouting, or it's a Deadly encounter if the enemies get the ambush off successfully. CR isn't the end-all-be-all of encounter-building, not even RAW.

[5eNeedsDarksun]

So, I am imagining an ambush encounter where five bandits have dug ten foxholes overlooking a road. They have a trenches that allow them to move from foxhole to foxhole and pop up and attack from being hidden. With surprise and some crossbows, a group of 1st level PCs could easily lose one or more members of their group.

Yet, if the wizard has an owl or hawk familiar scouting ahead, the bandits are easily detected and the group of PCs can avoid the ambush and even get the drop on the bandits by approach them from their blind spot.

One encounter - two vastly different outcomes. CR rating is the same for each.

You haven't really asked a question here. Some responses imply that you were looking for info on CR, but I'll respond as to the other part of your post: that depending on party actions and tactics the result could be much different.

To that I say great! That's a well designed encounter. At my table the party is rewarded for good preparation and scouting. If they just wander blindly ahead there are going to be encounters that they will struggle in and occasionally some that they'd be advised to avoid all together. Don't worry about it. And as for the implication of CR, whatever you do, don't award more XP for 'choosing' the more difficult option (blundering ahead and potentially getting surprised).