Skill

Skills are areas of expertise a character has points in. The higher the percentage in a skill, the better that character is in it. You receive more Skill Points by earning Levels through gaining Experience.

Skill advancement
Raising a skill one percentage point initially costs one skill point, or two percentage points for one skill point if the skill is tagged. When buying skill if the skill type is tagged, two skill % (at 1-100%) are given for each skill point spent. There are no half skill points for a cost of one. Similarly, when buying skills at cost two or higher, two or more skill points must still be purchased at once, for a cost of two, or three for three, etc. E.g. if a skill is at 101%, and tagged, the next purchase must be of two skill, taking the skill to 103%, for a cost of two skill points. To raise a skill one percentage point (or two if it is tagged) when its current percentage value is between or including: Note that if the second of the two skill points bought for a tagged skill ends up above the transition between two costs, the cost is the lower one. E.g. if a skill is at 100%, and tagged, spending one skill point takes the skill to 102%, for the cost of one skill point. Unlike perks, skill points can be left unspent and will remain, to be spent later. This could conceivably be useful in the event of a hurdle in the game of unknown type that would prevent further progress, e.g. a locked door. Any unspent skill points in excess of 99 are lost.
 * 1% to 100% costs 1 Skill Point
 * 101% to 125% costs 2 Skill Points
 * 126% to 150% costs 3 Skill Points
 * 151% to 175% costs 4 Skill Points
 * 176% to 200% costs 5 Skill Points
 * 201% to 300% costs 6 Skill Points

Real time 'Base' stat modifier to skills
SPECIAL stats (Strength, Perception, etc) modify skills with a considerable bonus. The game keeps track of this bonus throughout the game, in real time. Any changes to stats, (e.g. chem effects, aftereffects, and addiction effects, the Hubologist zeta scans, Gain Strength perk, Red memory module enhancement, Mirrored Shades equipped, ear bitten off by the Masticator etc) during the game add to or subtract from this modifier.

The effect of Difficulty settings on skills
Combat skills are unaffected by Difficulty settings Tagged skills are unaffected by Difficulty settings Active and Passive skills are lowered by 10 (%) when the difficulty is changed from Normal to Hard. Active and Passive skills are raised by 20 (%) when the difficulty is changed from Normal to Easy.

Fallout and Fallout 2 skills
The first six of the following skills are Combat skills The last four are Passive skills The rest are Active skills
 * Small Guns
 * Big Guns
 * Energy Weapons
 * Unarmed
 * Melee Weapons
 * Throwing
 * First Aid
 * Doctor
 * Sneak
 * Lockpick
 * Steal
 * Traps
 * Science
 * Repair
 * Speech
 * Barter
 * Gambling
 * Outdoorsman

Fallout 3 skills

 * Small Guns
 * Big Guns
 * Energy Weapons
 * Unarmed
 * Melee Weapons
 * Explosives
 * Medicine
 * Sneak
 * Lockpick
 * Science
 * Repair
 * Barter
 * Speech

Fallout Tactics skills

 * Pilot
 * Small Guns
 * Big Guns
 * Energy Weapons
 * Unarmed
 * Melee Weapons
 * Throwing
 * First Aid
 * Doctor
 * Sneak
 * Lockpick
 * Steal
 * Traps
 * Science
 * Repair
 * Barter
 * Gambling
 * Outdoorsman
 * Poop

Maximizing skill point gain / cost in Fallout 1/Fallout 2/Fallout Tactics
Reducing the base stat modifier of skills with chem effects, and/or by changing the difficulty to Hard, can reduce the cost of skill points if it lowers the skill below a skill cost threshold. E.g. Science can be raised to 135% for only one skill point per % by lowering Intelligence to zero with Mentats' aftereffects or Psycho effects (there is a limit of two chems unless you save and reload, but that will doubtless be necessary in any case to avoid become Addicted), and raising the Difficulty to Hard before spending. It can be raised a further 20% if needed, afterwards, by setting the difficulty to Easy (same goes for reading Books). This also works for higher tiers of skill point cost. Note, however, a player buying the higher tiers is doubtless in late game or finished with the game story, and may prefer to just wait for or look for the Vault City Designer Notes after finishing the oil rig.