Contrary to future generations, critical hits in Generation I do not occur at a fixed rate for all Pokemon. While the base rate is 6.25% (1/16) from Generation II onwards, in RBY, critical hit ratio is calculated uniquely for each Pokemon based on its base Speed (it cannot be lowered by paralysis or Speed drops, nor can it be boosted by Focus Energy, which suffers from a debilitating glitch). The formula is x * 100 / 512, where x is the base Speed of the respective Pokemon. For high crit moves (Razor Leaf, Slash, Crabhammer, and Karate Chop), the formula is x * 100 / 64; these moves have a crit ratio 8x higher than normal. The frequency of critical hits is an integral part of gameplay, with Pokemon like Tauros having a critical hit rate greater than 20%. However, critical hits ignore the player's stat drops and boosts as well as the foe's. This means that if a Mewtwo had used Amnesia to reach +6, it would do more damage with a normal hit than a critical hit.
Please note that results are rounded to the nearest two decimal places, and only fully evolved Pokemon are included.