As the orcs began to batter down the
door, the thief climbed the wall to the
safety of a ledge.  He wrapped 1 end of
a rope around a projection and threw the
other to the fighter left below.  The rope
dangled, swaying  a good 4 feet above
the fighter's head, as the door began to
splinter.

“Jump for it,” the thief yelled. “Then
climb.”

Both players looked at the Dungeon
Master.

“Do I make it?” the fighter said.

Groaning inwardly, the DM reached
for a heap of books. Play ground to a
halt. . . .
 

On page 110 of the AD&D™ Dungeon
Masters Guide, it is written that the DM
can control any situation not specifically
covered in the rules by assigning “a
reasonable probability for the event” and
letting the player roll dice in an attempt
to meet it. There are times, however,
when assigning this “reasonable probability”
leads to unreasonable results: arguments,
delays, and doubts. Often the
DM is baffled as to how to assign such a
probability. Players, quick as jackals to
pick up on signs of weakness of this sort,
argue that the probability is much higher
than the DM’s guess. Outlined below are
some guidelines, for players and DMs
alike, to help in setting such probabilities
for unforeseen events.

To begin with, even the most complex
game situation can be broken down into
its respective parts. At the root of all such
questions like our example is a simple
matter of success or failure, whose basic
parts are the character and the situation
in which the character finds him or herself.
The player character wishes to perform
an action successfully in a situation
where his or her prospects for success
(or degree of success) may be altered by
varying circumstances. To be successful,
the character performing an action
must use certain skills. If the DM keeps
these divisions of the game problem in
mind, his or her job will be much easier.

First, the DM must consider the makeup
of the player character. In deciding
whether or not the character can perform
an action, the DM should use that
basic part of the AD&D game system, the
character ability scores. Since each
score is a measure of aptitude for performing
certain kinds of actions, these
scores are the tools for pinning down
elusive probabilities for the success of
an action.

For instance, the intelligence score
shows the character’s aptitude for thinking
correctly and clearly. From this point
of view, the DM may turn the score into a
percentage chance for the character to
use the skill in question. Multiplying the
ability score by 5 gives a number we may
call the basic skill percentage.

Applying this to the ability scores of
the fighter in the earlier example, we get:
 

    Strength of 18, times 5 = 90% chance of using strength
    Intelligence of 8, times 5 = 40% chance of thinking correctly
    Wisdom of 9, times 5 = 45% chance of wising up
    Dexterity of 14, times 5 = 70% chance of manipulating objects
    Constitution of 17, times 5 = 85% chance of withstanding stress
    Charisma of 10, times 5 = 50% chance of persuading others

These basic skill percentages may be
applied to game situations in the following
way:

First, break the disputed action into
steps, if more than one specific action is
involved. In the example, the fighter
must grab the swaying rope, then climb
up it.

Second, determine which skill or skills
will be used in each step. Grabbing the
rope requires dexterity, and climbing requires
strength. Thus, the fighter has a
base chance of 70% to grab the rope
when he jumps and a base chance of
90% to climb up it (if the grab succeeds).

Often a situation arises where two or
more skills play a part in one action. To
get the base chance in these circumstances,
simply average the percentages
required, rounding up if necessary. In
the example, the fighter will need both
strength and dexterity to pull himself
onto the ledge once he’s made the climb;
70 + 90 divided by 2 = an 80% base
chance of scrambling over the ledge
successfully.

Once the DM has the base chance for
success, he or she must consider whether
the situation will modify that chance.
Obviously, adverse conditions make success
less likely, but it’s also possible that
certain conditions will favor success
beyond the base chance. Again, the DM
should keep the character’s ability scores
firmly in mind. If something in the situation
favors the use of that particular skill,
then the character receives a bonus.
Conversely, if something in the situation
interferes with the use of the skill, then
the character receives a penalty. After
adding and subtracting the situational
modifiers, the final percentage score will
be that to be met by the player’s roll of
dice.

Assigning these situational modifiers,
the most difficult part of the task, raises a
pair of important questions: the size of
each bonus or penalty, and how many
modifiers should be taken into account.
At this point, it is important for both DM
and players alike to remember that the
AD&D system is only a game, having,
like all games, arbitrary rules and limits
in order to remain playable. An important
part of playability is the speed at
which decisions may be reached. It is
possible to nit-pick over every tiny factor
in the situation and to assign bonuses
and penalties of widely varying percentages.
In the example, the DM might decide
that dust in the air makes the rope
harder to see, for a 2% penalty, but that
the rope is so thick that it is 13% easier to
grab. To do this sort of “adjusting” would
be not only tedious but pointless.

As far as the size of the bonus/penalty
increments goes, the DM should pick
one consistent with the rest of the game
system, then stick to it despite wheedling
from the players. Since many penalties
and bonuses in the AD&D rules
come in increments of 05% or 10%, a
logical choice would be to make normal
factors affect chances for success at the
05% rate and exceptional ones at 10%. If
our fighter were standing on slippery
ground for his jump, he would be penalized
-05%, but if he were up to his waist in
water, the penalty could justifiably be
increased to -10%.

When it comes to the question of
which factors to consider, a good rule is
to choose only those which have a direct
bearing on the skill being used, and of
those, only the most dramatic. In the example,
the fighter may be distracted because
orcs are banging down the door,
but such a distraction will have at best
only a dubious effect on his dexterity
and none at all on his strength. If, however,
he were trying to use his intelligence,
such a distraction would require
the assessment of a stiff penalty. The DM
should resist the temptation to consider
every possible liability in the situation
and figure it into the final score. On the
other hand, the player should resist the
temptation to find every possible detail
in his or her favor and demand the DM
include it. Consider: The fighter might
indeed have a slightly better chance of
grabbing the rope if he removes his
gauntlets, but then how is he going to
carry them up? The standard of judgment
should always be playability. Too
close attention to detail means a decision
that takes more time than it’s worth
and a boring wait for the other players.

Another general rule which the DM
should set in advance for these situations
is whether the PC’s
current HP total will have a modifying
effect on the basic skill percentages,
If the fighter has only one hit point left
out of 26, it might be that his effective
strength is no longer a true 18, On the
other hand, figuring hit points into this
kind of decision is somewhat contrary to
the spirit of the game. A fighter down to
one hit point still has the same chance to
hit on the combat tables. If the DM decides
to penalize for low hit points, the
justification can only be as a measure of
exhaustion and should be explained as
such to the players. Rather than using
some elaborate table or formula, the DM
who decides to include hit-point total
should set up a simple rule. A good one
might be that any characters down to
one-quarter or less of their maximum hit
points will suffer an automatic penalty of
05%, effectively equivalent to a temporary
loss of one point of ability score.

THE FORUM
I would like to comment on the current debate
over the most logical, realistic, and simplistic
system for determining the success of actions
attempted by characters, using the corresponding
ability score to generate a base percentage roll. It
seems to me that Katharine Kerr's system appeared
in an earlier issue of DRAGON Magazine.
[Editor's note: Yes, in issue #68.] I do agree
that her system is much more logical than the one
designed by Jonathan Heiles in his letter in issue
#91. <link>

I have come up with a subsequent system that
seems to follow a similar pattern as Katharine?s,
but which compensates for Jonathan?s complaint
that Katharine's system is too generous toward
low scores and too stringent toward high ability
scores. It also affords a variation of percentage
chances for scores between 15 and 18 inclusive,
unlike Kim Mohan's idea in his response to <link>
Jonathan's letter.

The system follows Katharine's 5% theory up
to and including an ability score of 10. This is
also the same as Kim?s suggestion, but this is
where my system changes. For scores between 11
and 17 inclusive, the adjustment should be an
additional 7%, not 10% as Kim suggested. This
would constitute a 99% chance to perform an
action which is taken into account by a score of
17. Thus at 18 (or higher in the case of strength)
the action being attempted would automatically
be successful, as it should be with a perfect ability
score.

Of course, everyone likes to create their own
system, but I?ve found that this system is easily
adapted to any campaign and is flexible enough
to be used by even the most devious DMs.

Steve Pajak
Schenectady, N. Y.
Dragon #94

*    *    *    *

A few comments on your answer to Jonathan
Heiles's letter in issue #91. <link>

The chance of rolling 5 or less on 3d6 is slightly
over 4.6%, not slightly over 3%.

The reason you couldn't think of an example of
a charisma check is simple: "Charisma check" is
simply another name for "reaction roll.? Any
situation that can be handled by either one could
be handled by the other equally well. The rules
specify the reaction roll table; so be it.

You and Mr. Heiles both missed the main point
of multiplying by 5 and using d100, namely that
there?s nothing sacred about the number 5. For
easy tasks where even low-attribute characters
have a good chance, you can multiply the attribute
score by, say, 8, 10, 15, or 20 (with 00 still an
automatic miss ? even a dexterity 18 character
can plant his foot on something that slips as he
makes his leap). For hard tasks you can multiply
by 3, 1, or ½. Dexterity times ½ on d100 is a
real test, yet even the clumsiest cleric has a
chance (unlike the reaction roll table, I might
add, where charisma 3 has no chance of a friendly
reaction).

By the way, some other FRP games (Bushido
and Runequest, for example) make quite extensive
use of attribute checks. It is even possible to
base the mechanics of a system on attribute
checks alone (see The Fantasy Trip).

Ralph Sizer
Providence, R. I.

*    *    *    *