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A Percentile Skill System for Traveller

This article was originally posted to the pre-magazine website in 2004, and reprinted in the January/February 2019 issue.

Traveller rawks, there’s no doubt about it. And you can get D6s pretty much anywhere. In fact at my parents’ house, all the board games in the cupboard are pretty well diceless from constant raids over the years.

But I’m a big fan of percentile systems, especially the Chaosium system, which I regard as one of the more elegant and simple mechanic sets in gaming today, especially in regards to skill advancement. So if you like it as much as I do then here’s how you can adapt Traveller to a percentile system.

Step 1: Assign base percentages: In blocks of 5% go through whatever skill list you are using and assign a base skill percentile rating. This is the basic chance that someone attempting a task with no real knowledge in that skill has in succeeding. For example, shooting a pistol is largely point and click (stuff about proper grip, flicking off the safety, etc., aside), so let’s give Handguns a base value 20%. Physics, however, is an ornery cuss that needs to be tamed by the mathematically inclined, so we’ll give it 0%. Actual values should be determined by the referee; in general, the easier it is for a character to use a skill to good effect with only a basic orientation, the higher the base percentage should be. The Base Skill Percentage table shows the values I’ve used when converting from MegaTraveller.

Step 2: Assign each skill a relevant stat grouping: Each stat grouping has a primary stat and a secondary stat, representing the stats most likely to have an impact on the use of the skill. The stat group provides a single modifier for rolls vs. skills in that group; “primary” and “secondary” indicate how much influence on the modifier the stat has. See the Stat Group table for determining which stats apply to which modifiers; the Base Skill Percentage table also shows the stat groups I’ve used when converting from MegaTraveller.

Stat Groups
Agility
Used for physically-focused skills that do not principally rely on fine motor control.
Example skills: Brawling, Blade Combat
Primary Stat: Dexterity (DEX) Secondary Stat: Strength (STR)
Charm
Used for interpersonal skills
Example skills: Liaison, Interrogation,
Primary Stat: Social Status (SOC)
Characters with Charisma (CHA)
instead of SOC use CHA instead.
Secondary Stat: Intelligence(INT)
Knowledge
Used for skills with an intellectual focus
Example skills: Computer, Navigation
Primary Stat: Education (EDU) Secondary Stat: Intelligence (INT)
Manipulation
Used for skills relying principally on fine motor control and coordination
Example skills: Mechanical, Electronics,
Primary Stat: Dexterity (DEX) Secondary Stat: Intelligence (INT)

Step 3: Calculate the character’s stat group modifiers: First, determine the average stat value (A) for each stat in the version of Traveller you are using. For standard humans in Classic Traveller, MegaTraveller, and Marc Miller’s Traveller (T4), the average stat value is 7 (2D); for standard humans in Traveller: The New Era, the average stat value is 5 (2D-2). Note that if you are attempting to apply this to races other than standard human, you should use the modified stat rolls for that race, e.g., for converting MegaTraveller Aslan, the average stat value for STR is 9 (2D+2)

Next, for each stat group, calculate the Primary Stat Modifier (P) and the Secondary Stat Modifier (S): Subtract the average stat value (A) calculated above from the character’s actual value for the stat. Preserve the sign; if the character’s stat is below the average, it represents a negative modification to the base skill percentage. For the Primary Stat Modifier (P), double the value thus calculated; for the Secondary Stat Modifer (S), use the value thus calculated. Add P and S; this sum is the Stat Group Modifier for that stat group.

For example, the ubiquitous Terry has a MegaTraveller UPP of 7A6864. The Average stat value is 7 (2D). Terry’s modifiers would be as follows:

Agility (Primary stat DEX, secondary stat STR):
Character DEX A(10) – average DEX 7 = 3, 3 2= 6;
Character STR 7 – average STR 7= 0; 6 + 0 = 6.
Terry’s Agility modifier is +6

Charm (Primary stat SOC, secondary stat INT):
Character SOC 4 – average SOC 7= –3, –3 2 = –6;
Character INT 8 – average INT 7 = 1; –6 + 1 = –5
Terry’s Charm modifier is –5

Knowledge (Primary stat INT, secondary stat EDU):
Character INT 8 – average INT 7 = 1, 1 2 = 2;
Character EDU 6 – average EDU 7 = –1; 2 + (–1) = 1
Terry’s Knowledge modifier is +1

Manipulation (Primary stat DEX, secondary stat INT):
Character DEX A (10) – average DEX 7 = 3, 3 2= 6;
Character INT 8 – average INT 7 = 1; 6 + 1 = 7
Terry’s Manipulation modifier is +7

Add the Skill Group Modifiers calculated above to the base chance percentage for skills in the relevant skill group.

Step 4: Convert current skill levels to % values and add to base skill chance (as modified by the stat group modifiers): Use the Skill Level Modifier table as shown. Add the indicated percentage to the modified base skill percentage calculated at the end of step 3.

Skill Level Modifiers
Skill Level Percentage bonus
0 +10%
1 +20%
2 +40%
3 +60%
4 +70%
5 +80%
6 +90%

Note: For each level beyond 6 add an additional 5% (although few referees would allow skills to get this high).

‘Serves As’ Skills: If a character has a skill that ‘serves as’ other skills, treat each skill in the combination separately. For example, in MegaTraveller, Liaison serves as Admin – 1 and Streetwise – 1. A character with a skill of Liaison-3 would add 60% to their Liaison percentage value and 40% each to their Admin and Streetwise percentage values (as though they had Admin-2 and Streetwise-2).

Example: Terry from the above example has Carousing-1. The referee (me in this case) decides this is an everyman skill and gives it a base value of 20%. I assigned Charm as the appropriate skill grouping. Terry’s modified chance looks like this
20% (Base Skill Percentage for Carousing) –5% (Charm modifier) + 20% (skill level 1) = 35%

Base Skill Percentages and Skill Groups
Skill Base Skill % Skill Group Skill Base Skill % Skill Group
Administration 10 Charm Intrusion 0 Manipulation
Artisan 10 Manipulation Jack-of-all-trades See note 1
Axe 10 Agility Jet-propelled Aircraft 0 Manipulation
Battle Dress 0 Manipulation Large Blade 10 Agility
Biology 0 Knowledge Large Watercraft 5 Knowledge
Blowgun 5 Manipulation Laser Weapons 5; 20 if tech 9+ Manipulation
Bola 0 Manipulation Leader 15 Charm
Boomerang 0 Manipulation Legal 5 Knowledge
Bow 10 Agility Liaison 10 Charm
Brawling 20 Agility Lighter-than-air Craft 0 Manipulation
Bribery 10 Charm Linguistics (see note 2) 5 Knowledge
Broker 10 Charm Mass Driver 0 Manipulation
Carousing 20 Charm Mechanical 10 Manipulation
Chemistry 0 Knowledge Medical 5 Knowledge
Combat Engineering 0 Knowledge Meson Guns 0 Manipulation
Combat Rifleman 10 Manipulation Mortars and Howitzers 0 Manipulation
Communications 10 Knowledge Naval Architect 0 Knowledge
Computer 0; 20 if tech 9+ Knowledge Navigation 0 Knowledge
Crossbow 10 Manipulation Persuasion 10 Charm
Cudgel  20 Agility Physics 0 Knowledge
Demolition 0 Manipulation Pilot 0 Manipulation
Disguise 5 Charm Plasma Gun 0 Manipulation
Early Firearms 5 Manipulation Polearm 5 Agility
Electronics 5 Manipulation Propeller-driven Aircraft 0 Manipulation
Energy Weapons 5 Manipulation Prospecting 0 Knowledge
Engineering 0 Knowledge Reconnaissance 10 Knowledge
Equestrian 10 Agility Recruiting 5 Charm
Fleet Tactics 0 Knowledge Rifleman 20 Manipulation
Foil 10 Agility Robot Operations 0; 10 if tech A+ Knowledge
Forensic 0 Knowledge Robotics 0 Knowledge
Forgery 0 Manipulation Screens 0 Manipulation
Forward Observer 0 Knowledge Sensor Operations 0 Knowledge
Fusion Gun 0 Manipulation Ship Tactics 0 Knowledge
Gambling 10 Charm Ship's Boat 0 Manipulation
Genetics 0 Knowledge Sling 0 Manipulation
Grav Belt 0; 5 if tech B+ Manipulation Small Blade 15 Agility
Grav Vehicle 0; 20 if tech 9+ Manipulation Small Watercraft 0; 5 if hydro 4+ Manipulation
Gravitics 0 Knowledge Spinal Weapons 0 Manipulation
Guard/Hunting Beasts 5 Charm Stealth 15 Agility
Handgun 20 Manipulation Steward 20 Charm
Heavy Weapons 5 Manipulation Streetwise 5 Charm
Helicopter 0 Manipulation Submachinegun 10 Manipulation
Herding 5 Charm Survey 0 Knowledge
High Energy Weapons 0 Manipulation Survival 0; 10 if atmos 4-9 Knowledge
High-G Environment 0 Agility Tactics 5 Knowledge
History 15 Knowledge Tracked Vehicle 5 Manipulation
Hovercraft 0 Manipulation Trader 5 Charm
Hunting 5 Knowledge Turret Weapons 5 Manipulation
Instruction 10 Charm Vacc Suit 0 Agility
Interrogation 5 Charm Wheeled Vehicle 0; 10 if tech 4-8 Manipulation
Interview 10 Charm Zero-G Environment 10 Agility
Note 1: J-O-T is harder to convert. I recommend the following. Each session the character is allowed (J-O-T level) automatic re-tries at a failed skill roll without having to make a determination check.

Note 2: For each level of Linguistics the character receives their EDU % stat value in a specific language, e.g., Terry has Linguistics-2 and can speak Vilani and Gvegh. His EDU % value is 45%. He gets 45% in Vilani and 45% in Gvegh. From this point he advances in those languages normally. Once converted, Linguistics no longer offers automatic language ability. Instead, if a character’s Linguistics skill % is higher than a language %, they double any skill advancement rolls.

Skill Resolution: It’s a simple matter of if a percentile dice roll is equal to or less than the character’s percent chance of success as calculated above, they succeed. This assumes a difficult task. For easier tasks modify the chance by blocks of 10% upward. For example, a routine task might be +20; a simple task might be +40%. For stupidly harder tasks, modify in blocks downward, e.g., a formidable task might be –20%.

Experience: This is the best part. The Traveller experience systems were always pretty harsh on the old character. And, let’s face it, players love nothing more than advancing their character’s skills. While we mortals spend our leisure time watching TV, or gaming, our fictional counterparts are hitting the gym or the books, watching what they eat, etc.

Again, borrowed from Chaosium, experience is simple. If a character succeeds at a skill during a game session that the referee feels advanced the story and so forth, they can check it. At the end of the session they roll percentile dice. If the result is equal or exceeds their current skill then increase the skill percentage chance value by D10/2 (round down; range is 0-5%). The beauty is that probability means that higher skill percentage chances will be much harder to obtain.

Training: Once a PC spends (current skill %) hours dedicated to training in a specific skill they can make a roll vs. INT. If they succeed, they get a skill check, as for experience. The referee is free to modify the roll for access to instructors, training aides, conditions or study etc.

Statistic percentile checks: Naturally, if you’ve gone and applied the above, you'll want to do the same for stat checks.

For Classic Traveller, MegaTraveller, and Marc Miller’s Traveller, where the average stat value is 7, add 3 and multiply the result by 5. e.g., 7+3 is 10; 10 5 is 50%. For Traveller: The New Era, where the average is 5, multiply by 10, e.g., 6 10 is 60%.

Advancing Stats: If a character checks a stat, make the experience roll as normal, adding any percentage boost to the stat percentile value. When their stat percentile value reaches a multiple of 5, increase the actual stat value by 1 point.

Example: Terry has an INT of 8, with a percentile value of 55%. He manages to get 3% added to his INT percentile chance. So, his stat now reads INT 8 (58%). When his INT percentile chance gets to 60%+, he can boost the actual value of his INT from 8 to 9 (his stat would then read INT 9 (60%)).