How does the poisoned condition affect the challenge rating?

Question

I am planning an encounter against different Ogres (Half-Ogre, Ogre, Ogre Battering Ram...). During the fight, the Ogres are drunk and suffer the poisoned condition (disadvantage on attack rolls and ability checks). How would the poisoned condition affect the challenge rating of this encounter? Can I just halve the challenge rating since the disadvantage on the attack rolls will around halve the expected damage?

And as a follow-up question: Is there an easy way to determine the effects of different conditions on the challenge rating? I assume the disadvantage on attack rolls is in my encounter (with the Ogres doing the attack rolls) much more relevant than it would be in encounters against a wizard where the party would need to do some saving throws.

Answer 1

You decrease the difficulty class of the encounter by one step

Circumstantial conditions do not affect the CR of the monsters, they modify the difficulty class of the encounter. This is treated on p. 85 of the DMG:

Increase the difficulty of the encounter by one step (from easy to medium, for example) if the characters have a drawback that their enemies don't. Reduce the difficulty by one step if the characters have a benefit that their enemies don't. Any additional benefit or drawback pushes the encounter one step in the appropriate direction.

The characters have a benefit their opponents do not: they are sober.

So if that encounter would have been deadly with sober ogres, you instead treat it as hard.

Because of this there is a very easy way to determine the effects of different conditions on CR: there is no effect on CR. CR is a fixed number that does not change. What you do change in encounters are the XP values for the CR based on number of monsters, and encounter difficulty, based on circumstances.

As an added note, both CRs and the rules for calculating encounter difficulty are notoriously inexact. For example an encounter with 3 or with 6 creatures of a given CR has exactly the same encounter multiplier of x2 (p. 82 DMG) and overall XP value, even though by all reason the encounter with twice the number of opponents should be twice as hard. So, don't sweat those CR calculations. Focus your precious prep time on something else instead.

Answer 2

Ogres are pretty much CR 2, regardless of the condition

Challenge rating is tricky. Albeit we have a generic table for CR calculations, the final value is mostly empirical. For instance, Orge is CR 2 because of its one-shot damage:

the ogre is a CR 2 creature because its damage is enough to drop most 1st-level characters in a single hit

Disadvantage on attack rolls doesn't change this much, so this CR still should be 2.

Challenge rating is also deceitful. It gives you a feeling of "balanced" (hopefully not boring) encounter, while the real outcome can be very situational. In the end of the day, building good encounters requires knowlegde and experience. I suggest reading The Angry GM blog as a starting point:

• Four Things You’ve Never Heard of That Make Encounters Not Suck
• How to Build F$&%ing Awesome Encounters!
• How to F$%& CR: A Practical Example of Encounter Building the Angry Way