Critical chance rolls?

What's the easiest way to implement a critical chance roll? I want to have a global variable for the player that can be upgraded as the game progresses.

Give your player an instance variable (called CriticalChance ?) and give it a value (1 ?) then you can put something like ..

if player hits then damage = 10

if random(100)<chriticalchance then damage=damage *2

take damage from baddies hitpoints

?

as player puts experience into critical chance (player goes up a level)

make (variable) CriticalChance =CriticalChance+1 (or 5 or 10)

?