Thursday, August 28, 2008

Exploding Dice

Some RPGs have a mechanic called "exploding dice" - in general, it means that if a die rolls it's highest value, you roll it again and add both rolls together. Example: You roll a "6" on a d6 - in a game with exploding dice, you'd roll the 6 again and add both rolls together. In some versions, a second "6" result would mean a third roll and a result greater than 12.

Most people who read this blog are probably familiar with the concept and the term, but I'd rather just explain the reference here rather than have to explain it again every time I mention the term.

One interesting side-effect of exploding dice is that certain results can't be rolled, and your bell curve ends up lop-sided. Using the d6 example above, you can roll 1-5, 7-11, and 13-17, but you can't ever roll a 6, 12, or 18.

This can have major impact if your gaming has a sliding difficulty scale. If the difficulty was 5, and you apply a standard +1 difficulty modifier, you've just cut the chance of success in half (from 33% to 16.6%). However, a second +1 difficulty modifier on the same roll doesn't impact the chance of success at all (stays at 16.6%), and a third +1 difficulty modifier only reduces it slightly (from 16.6% to 13.8%).

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