Passive skills are lowered by 10% when the difficulty is changed from Normal to Hard. 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) during the game add to or subtract from this modifier. It is thus possible to reduce any combination of stats to a minimum value, significantly lowering skill values.
This way it's possible to raise Small Guns to 100%, First Aid, Repair and Outdoorsman to 110% and Science to 119%, in the case of a player whose Perception, Intelligence and Agility are at 10. To lose Agility and/or Endurance, use Buffout, then wait six hours. To lose Intelligence and/or Perception, use Mentats and wait a day. To lose only Intelligence use Psycho. To lose only Perception, drink beer, booze, etc. If books are used from inventory (by left clicking and dragging) rather than from a weapon slot, time will not pass, and therefore stat effects will not wear off, until the inventory screen closes.