The Ninja perk grants you the power of the fabled shadow warriors. When attacking, you gain a +15% critical chance on all Melee and Unarmed attacks. Additionally, Sneak attack criticals do 25% more damage than normal.
pcps3xbox360 In Fallout: New Vegas Ninja multiplies your critical chance by 1.15 instead of adding a flat 15%, unlike all other perks/traits that modify critical chance. For example, a character with a critical hit chance of even 25% would have it be increased to only 28.5% rather than to 40% as it should. (25% * 1.15 = 28.75% vs 25% + 15% = 40%) [verified]
pc In Fallout 3, the extra 15% critical chance does not work without the help of the unofficial Fallout 3 patch. [verified]