Posshum V1.2

.:POSSHUM v1.2:.

a role -playing game system J. Eicher and D. vd Merwe

The 'Probability Of Success using a Standard Human Unit Measurement' system was designed to give the DM and players greater control over the difficulty and outcomes of complex tests of skill. Based on the popular D20 system of 'Difficulty Classes' it seeks to provide the same ease of use while being more expansive in its application and achieving greater resolution. The basic POSSHUM approach to establishing the difficulty of a specific action is by multiplying component probabilities together. To do this however a 'Standard Human' has to first be defined as a healthy 25-year old male in the year 2008 who has experienced a 'normal' and simple upbringing having the full scope of normal human physical, emotional and mental capabilities.

A 'Standard Human' is thus considered to be able to respond to all challenges as a normal healthy person in their mid-twenties could and is functioning at 100% 'Standard Human'. Characters should be created by modifying a 'Standard Human'. That is, starting with a value of 10 (or 100%) for all the Base Attributes. Addition of modifying skills/feats can affect Base Attributes making the person into something more than 'Standard'. The players do not get to roll for attributes nor is it necessary to hand out a number of Attribute points for a player to assign as he pleases (thou of course the DM could do this if he wanted to create a party of highly powerful characters). A character who through training/serendipity/genetics/augmentation attains an attribute at 200% is responding to situations governed by that attribute with double the proficiency of a 'Standard Human' (e.g. a person with 200% Charisma will be able to act, at their best, with twice as much charm as a normal human). As a character's attributes grow beyond this point they delve further into the life of a 'Super Human'. A . Establishing the 'RDR '. For the first example we'll use the simple case of a Standard Human, say a healthy 25-year old male (said to be operating at 100% normal human capability) jumping over a hurdle. Now this shouldn't be a problem for a healthy person in their mid-twenties and we'd probably be justified in saying that one such person should have a 1 in 10 chance of failure taking all possible extenuating factors into account (e.g. wind, loss of traction, a momentary distraction). Thus, the 'Standard Human' has a 0.90 probability of succeeding at jumping over the hurdle, let's call this the POSSHUM value. Using 2d20* this works out to a Required Dice Roll or 'RDR' value of 4 * 2d20 gives greater resolution than 1d20. In the case of a Standard Human having anywhere from a 1% -2.5% chance of fulfilling a task using a single d20 would require a DM to round off the RDR value to 20. However a character that has say, a 4% chance of completing a task suddenly has to roll at least 19 on a d20. That is, rolling 19 or 20 is considered a success (0.96 x 20 = 19.2), which is 10% chance and far more lenient than 4%. The 2d20 system however (after rounding off) would require the player to roll at least 38, which is a far more realistic 5% chance of success.

(40 x (1-POSSHUM)). In this case the average 25year old character would be required to roll at least a value of 4 on a 2d20 roll to successfully clear the hurdle. If however, the character had to perform another action while jumping over the hurdle like taking a photograph, we have two probabilities to deal with. Let's say that the 'Standard Human' has a 1 in 15 chance of messing up an individual photograph (e.g. by cutting out someone's face, putting their hand in front of the lens). This corresponds to a POSSHUM value of 0.93. As the actions of jumping over a hurdle and taking a photograph are independent actions and their associated POSSHUM values take into account all extenuating factors involved in the respective skills we can multiply the two values (which are just probabilities) together and used in the following equation to calculate RDR: RDR = 40×1−Hurdling POSSHUM × Photography POSSHUM  = = = ≈

40×1−0.90×0.93 40×1−0.837 6.52 7

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Therefore, a 'Standard Human' would be required to get at least a 7 on a 2d20 roll to succeed at hurdling and photography simultaneously. B . Applying character mods . The POSSHUM values are calculated for a 'Standard Human' but what happens if the character being played is not 'Standard' (a healthy 25-year old male)? In this case the character's percentage of reacting appropriately in a given situation is modified either in the positive, to represent things such as training in a specialised field or an outstanding base ability (e.g. a Dexterity higher than 10, which is considered standard) or in the negative in the case of handicap. The previous example calculated the Probability Of Success for a Standard Human to take a photograph while hurdling. Let's start with the simple case of jumping over a hurdle only and use a character who is a trained athlete. This person would very likely be responding to this challenge at 130% of a Standard Human and would in fact multiply his 2d20 roll appropriately: The RDR for jumping over a hurdle is 4, thus our new character makes his 2d20 roll, let's say he gets 10. This value is then multiplied by 130% (or 1.3) yielding an effective result of 13. In this way his exceptional athletic ability is factored into the calculation and he succeeds by a large margin. In this scenario the player has to roll at least 4 on a 2d20, anything less is considered a critical fail but we shall see how additional complications can make even trivial actions difficult. To illustrate how the POSSHUM system works with complex characters as well as complex situations let's give our star athlete a handicap. Let's say he has Parkinson's disease which reduces his photography skill significantly down to 40%. A normal, healthy 25-year old we said has a POSSHUM of 0.93 for taking a photograph and in a second scenario, photographic hurdling, we multiplied the POSSHUM values for hurdling and photography together and calculated an RDR of 7. The same thing can be done with the character's attributes that are being tested to determine how to modify the 2d20 roll: In this case our character has exceptional athletic ability, 130% of a Standard Human but unfortunately his photographic ability is handicapped at 40%. Thus by multiplying the two values together we get the probability of our tragic hero's success at taking a photograph while jumping over a hurdle, that is, 52% or 0.52. Therefore

with an RDR of 7 the player now has to roll at least 13 on a 2d20 to succeed (7 / 0.52 = 13); anything less will result in the character either being distracted, or losing traction and slipping, or dropping the camera in mid-spring or any other invention of the DM. In the second scenario, though still fairly trivial, it has in fact become significantly more difficult to for the character to complete the task at hand. The POSSHUM system models reality nicely because just as it is almost twice as difficult for the player to roll a 13 instead of a 7 (the RDR for a Standard Human with 100% athletic and photographic ability) it is arguably twice as difficult for an athlete afflicted with Parkinson's disease to jump over a hurdle and successfully take a photograph. C . Determining constituenT probs . and mods . One can clearly see how the POSSHUM system can be used to model the simplest and even most complex scenarios by multiplying together constituent probabilities. One obvious hurdle (as it were) to overcome is how to determine the difficulty of constituent actions and the effect of modifications to the character. For instance, how are we to know that a Standard Human has a 1 in 15 chance of failing at hurdles? Dealing with difficulty and mods. A large part of deciding on the difficulty of individual actions is at the DM's discretion but can be made significantly easier by dividing the constituent actions between three classes. The first class we shall call Base Ability Actions (BAA). This class contains all actions that are not governed by any skill in addition to the 6 standard ability scores: Strength, Dexterity, Intelligence, Wisdom, Constitution and Charisma. An example of the first class would be jumping over a hurdle. The action of jumping is inherent to humans, any normal person can do it. The second class we shall call Skill Ability Actions (SAA) and it contains all the actions that require specialised training in a skill. The third class, Intuitive Ability Actions (IAA), is for all the sundries, those abilities that are quirks provided by an intuitive ability, genetics or anything in addition to the base abilities or academic study. These actions usually grant the character an exceptional ability that doesn't require rolling the dice such as 'Combat Reflexes' which provides additional attacks of opportunity. Earlier we said that a person trained in athletics would be far better at hurdles than the Standard Human, in fact we estimated that he would be 130% better. Now the d20 system has a collection of skills and feats which are abilities and ability modifiers respectively. Skills are learned abilities that enable the character to perform actions which a Standard

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Human would not be able to. Feats modify either Base Abilities or Skills for example: a person with the 'Heal' skill is able to practice field medicine while a Standard Human couldn't, the 'Self-Sufficient' feat grants the character a +2 on heal checks. The rule of thumb is that each modification point in d20 is the equivalent of 10 % (or 1 point) in a POSSHUM game. A player begins with a Standard Human and adds skills/feats/handicaps to modify the character. So the 'Self-sufficient' feat which grants +2 on heal checks in the d20 system is the equivalent of 20% in POSSHUM. For the sake of simplicity in POSSHUM a feat such as 'Athletic', which grants a +2 bonus on Climb and Swim checks would merely grant the character a +2 to Dexterity because the Standard Human can both Climb and Swim (at least, being taught to swim is a part of a standard modern upbringing). Whereas the skill 'Pilot 1' would grant a Standard Human the ability to pilot light aircraft. This would bring the character's piloting ability up from about 5%* to 100%. Any advanced training in piloting an aircraft would modify the character's piloting ability. In this way we can simplify our previous scenario by allowing the character's simultaneous actions of jumping and photography to be governed by his Dexterity which would be modified in the positive by his athletics training (up by 20%) and in the negative by his Parkinson's (down by 40%). The final value for his Dexterity would thus be 80%. Character Mods . If our character happened to have advanced athletics training specialising in hurdle-jumping we could say that his athletic skill which was initially at 120% (because he had the Athletic feat) gets pushed up to 140% in the specific case of hurdling. Thus, whilst jumping over a hurdle and only for the application of jumping over hurdles the character's DEX score increases by 20%. Note that this would be at the DM's discretion. When an action demands only a learned skill (of which the first level usually grants the character 100% ability) the DM does not take Base Attributes into account: the random fluctuations of character and environment are governed by the role of the dice (e.g. Piloting a light aircraft through an alpine valley * The reason an untrained person is granted a 5% proficiency in flight is that a Standard Human who can drive a car arguably has a 1 in 20 chance, given enough time, to start a plane and get it off the ground. Of course, sustained flight, aerial manoeuvring, responding to weather patterns, aeronautics, navigation, landing etc. should very appropriately be designated a significantly lower chance of success. 0% proficiency is reserved only for those cases in which it is utterly impossible for the character to fulfill the task at hand (e.g. he is a quadriplegic).

would be governed by the character's 'Pilot' skill and only that skill as the skill encompasses his ability to perform all the functions relevant to the skill of piloting). The tables below offer a helpful guide to character modifications but bear in mind that the values are at the end of the day entirely at the DM's discretion, these values have been tested and work in a real game situation. The following table is incomplete and provided as a guide for DMs. D . Combat . The basic combat system in POSSHUM is based on the simple 'roll-against-roll' technique using a single d20. The attacker rolls an attack-roll, the defender a defence-roll. Both rolls are modified by BAA, SAA, IAA, Armour Class and Base Attack Bonus. The only adjustment required is that the NPC opponents out of a book like the Monster Manual might need to be adjusted relative to the Standard Human scale. E. Criticals. In POSSHUM critical rolls for actions other than combat are excluded. The probability of rolling 20 on both d20s is

1 1 1 × = 20 20 400

which is a

significantly lower probability than 1/20 on a single d20 roll. One calculates the equivalent probability as follows, where r equals the roll required on both dice to yield a probability of 1/20 (or 0.05%): 1 r r = × 20 20 20 2 1 r =  20 20 1 r = 20 20 1 r=20 =4.47 20

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Thus, '4.47' or less has to be rolled on both dice for a critical failure and equal to or higher than '20 – 4.47 = 15.53' on both dice for a critical success. If we round this number down to 4 it yields a 1/25 chance of critical failure or success rolling 4 and below or 17 and above on both dice respectively. Rounding up to 5 yields a 1/16 chance of critical failure or success rolling 5 and below or 16 and above both dice respectively. These numbers do not adequately approximate the 1/20 chance of criticial achieved by a single d20 roll neither do they provide sufficient resolution as a characters will succeed at performing extremely difficult actions with far too much leniency. For instance, an action that has a 2% POSSHUM value would be automatically achieved by a Standard Human character who scores a critical success roll of 16/16 on a 2d20. This is the equivalent of increasing the probability of success to 20%! In fact, any time a player rolls at least 20% on 2d20 he succeeds. For this reason critical rolls are only retained for combat in POSSHUM.

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BAA Modifiers Adjustment SAA Modifiers

Adjustment

IAA Modifiers

Adjustment

Acrobatic

+2 DEX

Armour proficiency (light)

100% light armour

Augment Summoning

Summoned creatures gain +2 STR, CON

Agile

+2 DEX

Armour proficiency (medium)

100% medium armour

Blind-fight

Re roll miss chance for concealment

Alertness

+2 WIS

Armour proficiency (heavy)

100% heavy armour Combat Casting

Athletic

+2 DEX

Deceitful

+2 CHA

Improved Disarm

+40% on disarm attempt

Deft Hands

+2 DEX

Improved Feint

Feint as move action

Combat expertise

+20% bonus on Concentration checks for defensive casting Trade attack bonus for AC (max 50%, which is 5 points)

Improved Trip +40% on trip attempt Whirlwind Attack

1 melee against each opponent in reach

Combat reflexes

Additional attacks of opportunity

Dodge

+10% dodge bonus to AC Mobility

+40% dodge bonus to AC against some attacks of opp.

Spring attack

Move before and after melee attack

ETC. Modified from: The Dungeons and Dragons Player's Handbook – Core rule book v3.5. (Groot-Bijgaarden, Wizards of the Coast, Belgium. 2003), p. 90.

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