Fallout skills

Skill advancement
Each skill has its own skill level which is expressed as a percentage. Skill levels can be increased by allocating skill points earned from gaining levels. Skills can also be improved by reading specific books or completing quests within the game.

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 skills, 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. For example, 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:
 * 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

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. For example, 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.

Effect of stats on skills
SPECIAL stats 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 or Mirrored Shades equipped) during the game add to or subtract from this modifier.

Effect of difficulty settings on skills

 * Combat and 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".

Maximizing skill point gain/cost ratio
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. For example, 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 doubtlessly 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.