Fumble Probabilities
v0.2 UPDATE BASED ON COMMUNITY FEEDBACK
BACKGROUND

Weapon fumbles can make the game more fun, however, if a weapon fumble is based off a static probability per attack (e.g. a '1' on 1d20) then a high level character is much more likely to fumble than a low level character because they have more attacks per round. Why would a 20th level fighter fumble more often in a fixed time period than a 1st level wizard, when the fighter would have much more skill with a weapon?

The risk associated with fumbling comprises two components:

• Probability of a fumble occurring
• Consequences if a fumble does occur

This planned extension is designed to model a reduction in fumble probability as a character becomes more experienced / skillful. Fumble consequences are out of the scope of this extension to allow for GMs to choose from the options available.
TUNING OBJECTIVE

20th level fighter should have an overall probability of approximately 1 in 400 of fumbling (0.25%)
1st level wizard should have an overall probability of approximately 1 in 20 of fumbling (5%)
PROPOSED CHANGE TO FUMBLE

1. An attack roll of a '1' indicates that the attack missed and that a fumble is possible
2. A second check (the ‘fumble check’) is taken to see if the fumble occurs, vs DC 20. There are seven potential adjustments to the check(1-7. below):

1. + proficiency bonus
2. -2 if not proficient with weapon
3. + dexterity bonus
4. each attack action per round > 1, +5 (e.g. a 5 for 2 attacks per round)
5. magic weapons +1 per +1 of the weapon
6. light weapons +1
7. heavy weapons -1

EXAMPLE 1
A Nat 1 has been rolled and the player has a 20th level fighter with 12 dexterity, wielding a +2 two handed maul who rolls a fumble check vs DC=20.

Modifiers
d20 check

1. proficiency bonus (+6)
2. non-proficiency penalty (0)
3. agility bonus (+1)
4. each attack action per round >1 (+15)
5. +1 per +1 of the weapon (+2)
6. Light weapons +1 (0)
7. Heavy weapons -1 – a maul is heavy (-1)

• TOTAL modifier=+23

Player rolls a d20 +23 vs DC20. This means the character only fumbles on another Nat 1 (5% probability of failing fumble check; resulting in a 0.25% probability of fumbling).

EXAMPLE 2
A Nat 1 has been rolled and the player has a 1stlevel wizard with 14 dexterity, wielding a dagger who rolls a fumble check vs DC=20.

Modifiers
d20 check

1. proficiency bonus (+2)
2. non-proficiency penalty (0)
3. agility bonus (+2)
4. each attack action per round >1 (0)
5. +1 per +1 of the weapon (0)
6. Light weapons +1 – a dagger is light (+1)
7. Heavy weapons -1 (0)

• TOTAL modifier=+5

Player rolls a d20 +5 vs DC20. This means the character fumbles on 1-14 on a d20 (70% probability; 3.5% probability of fumbling).

CONCLUSION

The fumble probability came close to our ‘tuning objective’ set above.
20th level fighter

• Target: 0.25%
• Actual: 0.25%

1st level wizard

• Target: 5.0%
• Actual 3.5%

Overall the proposed system reduced the probability of fumbling for low level character by a little bit, but makes the probability of fumbling for high level characters very rare indeed.

Main changes
Changes since v0.1

• Created a fumble check vs DC20 rather than a set of modifiers to create a unique DC per character.
• Added a ‘weapon proficiency penalty’ modifier if a character has no proficiency with the weapon (thanks deer_buster for the suggestion)
• Demonstrated, using two scenarios, the effect of the new rule on the overall probability of fumbling compared with a tuning objective