Scaling up Difficulty Class (DC) on successive checks is a bad idea. Some DMs like to increase DCs on successive attempts, dissatisfied with the relative ease of a standard skill check (or saving throw).
Consider the following examples where an increasing DC might be used:
- A climbing expedition where the party ascends to a blustery summit.
- A Dwarvish drinking contest where the participants drink more fortified ales in successive rounds.
- A sealed magma-filled chamber where the heat continues to intensify.
- A raft capsizes, dumping the party into a river. They need to swim to shore as the current picks up closer to the impending waterfall.
Using these escalating checks has two drawbacks:
- The odds of passing the skill check drop too fast.
- The skill check eventually becomes impossible.
I’ve made a calculator/visualizer that shows these effects at play:
You can see the odds of success on this chart, represented by the turn-by-turn and cumulative lines. Even when the turn-by-turn is steady, the cumulative odds of success drop quickly. This shows that it’s not necessary to increase the DC to achieve the desired effect.
Odds of Success
Think of it like this: by rolling against the same check twice, you’re already rolling with disadvantage. You need both those checks to pass. This cumulative effect is punitive enough without increasing the DC.
Easy Check. A character with a +5 modifier may pass a DC 10 skill check 80% of the time. However, on the second try, only 80% of those 80% who passed will continue, meaning only 64% will have passed both skill checks. By the third check, the odds are 51%–a coin flip.
Medium Check. A character with a +5 modifier may pass a DC 15 skill check 55% of the time. On the second try, only 55% of those 55% who passed will continue, meaning only 30% will have passed both skill checks. By the third check, the odds are 17%. After the fourth check only 9% remain.
Hard Check. A character with a +5 modifier may pass a DC 20 skill check 30% of the time. On the second try, only 30% of those 30% who passed will continue, meaning only 9% will have passed both skill checks. By the third check, the odds are 3%–less than a critical hit.
So we see the odds of passing successive skill checks already falls off very quickly. It’s not necessary to increase the DC on successive rounds to get this effect. If you do increase the DC, you’re just making the odds of successive successes minimal.
Your natural inclination may be to increase the check from Easy (10) to Medium (15) to Hard (20). That’s an increase of +5 for each additional skill check. Let’s see what happens when you do that:
This shows that a drop in the turn-by-turn odds will be magnified even more with respect to the cumulative odds of success. This shows that increasing the DC is likely to have a greater effect than intended.
Let’s assume you realized this and are using a more graduated DC increase. Since the smallest possible increase is +1, let’s take a look at that:
Here you see than even when the turn-by-turn odds have a nice, gradual decrease, the cumulative effect is pronounced. Where you may have intended the character to have a 30% chance of failing the check on the third try, the cumulative risk is nearly twice that, at 58%. Keep in mind that this is the least pronounced increase against the easiest DC.
Impossible Skill Checks
Another effect of increasing DCs is that you quickly make the DC impossible. If you start with a Medium DC (15) check and increase it one tier (+5) every round, then it will be impossible to pass on the third round without the assistance of modifiers. Even then, you will need a +5 and a natural 20. At some point, the DC increase will outpace even the modifiers, making the check impossible to pass. The larger the increase, the quicker the task becomes impossible.
Mitigating DC Check Failure
There are ways to manage the increasing chance of failure resulting from making a check every round. Ultimately these methods work to ensure that the first failure is not fatal.
Tone Down Penalties
If you’re going to fall off the cliff when you fail a check, it’s a black-and-white proposition. Failure is death. Consider employing environmental factors that will damage your players without destroying them. Instead of fighting on a gusty precipice, maybe it’s a magma-filled chamber that deals a little heat damage and inflicts escalating levels of exhaustion.
Allow Some Failure
In a skill challenge, the party is permitted a certain number of failures while preserving the odds of an overall success. For example, a party may need to gain five successes before three failures to escape a collapsing tower.
While I recommend setting different DCs for various tasks for a skill challenge, a cumulatively increasing DC can still be quite punitive. If you are going to use increasing DCs for a skill challenge, increase gradually, and budget failures accordingly. I have a lot more to say (and some other tools to share) on the topic of skill challenges, so let’s save that discussion for another day.