D&D To HOW Stat Translation

This is just a quick recap of translating D&D attributes into their Heroes & Other Worlds equivalents, if what you care about is how much the attribute adds to the chances of success on a roll.  See my previous post on Bell Curve vs. Linear for a deeper explanation of the probabilities involved.   It seems to me that chance of success is probably the best way to look at it, since that’s most often what you’re going to be directly testing, at least in HOW.  There are other things the attributes are good for (e.g. carrying capacity for STR in D&D), but they seem to me to be relatively minor compared to adjustments to your chance to hit, say.  In D&D, particularly old D&D, attributes are much less important than in HOW: the difference between a 12 and a 9 in Basic D&D is almost purely cosmetic, and even the difference between a 3 and an 18 is no more than the difference between a 10 and a 13 in HOW when it comes to applying the bonuses to a d20 roll.

old D&D New D&D HOW
2-3 8
4-5 8-9
3 6-7 9
4-5 8-9 9-10
6-8 9
9-12 10-11 10
13-15 12-13 10-11
16-17 14-15 11
18 16-17 11
18-19 11-12

If the HOW column shows a range, use the lower number if you’re at the lower end of the D&D range, or the higher number if at the higher end.  Flip a coin if the number is exactly in the middle.

Update: Since Tim Knight asked, I’m including a little more of the reasoning behind having the D&D stats represent such a tight range of HOW stats… is a stat over 12 in HOW really superhuman?

D&D stats add comparatively little to your abilities.  Even in the more generous editions, an 18 is only a +4 bonus… which is +20% on a d20 roll. Starting from a base to-hit of 50% vs. unarmored foes, that gives you a 70% chance of landing a blow.  In HOW a score of 12 gives you a 75% chance of landing that same blow.

There are other ways you could look at it.  For instance, as a straight roll under stat to see if you succeed, an 18 is 90%, which is equivalent to a HOW score of 14.  But by-the-book D&D never employs rolls like that. Instead, where stats matter at all, it’s almost always as the tiny (+/- 20%) bonus.

The systems have pretty different underlying assumptions of competence, but it seems to me that matching bonuses as I did tends to minimize “system shock” where translating a character from one to the other makes it vastly more or less likely to succeed at tasks than in its home system.   A beginning HOW character is much more likely to succeed than a beginning D&D character; they’re much closer in competency to a mid-level D&D character, even though HOW being a much more deadly system overall tends to make them feel comparatively fragile.  E.g. a beginning thief in D&D has only 15% chance of picking locks or 20% of picking pockets compared to the 50% a 10 DX thief has in HOW. The one thing that D&D characters get a lot better at over time is taking multiple blows, though I haven’t yet tried to factor in the difference between armor as DR and armor as deflection