Critical Hits are attacks that cause extra damage in many games that use health, or some other numerical unit that represents victory and defeat. The recipient of a critical hit will typically take an amount of damage that exceeds the normal damage value range of the attack. The chance of scoring a critical his is in general dependent on a variety of factors defined by the games in which they can occur.
Critical hits introduce a level of uncertainty to an exchange of blows, giving the conflict in games that use critical hits a more natural feel which closer approximates the vagaries of real-world combat. In almost any conflict or altercation that can be translated into a game, some attacks are much more telling than others. The visual depiction varies from game to game, from screen-shaking, red flashing, extra-large and colorful damage numbers, extra blood, and cries of pain from the game character on the receiving end. The in-game meaning of a critical hit varies as well, from a blow to the head to a particularly powerful burst of energy in a magic spell. Some games feature abilities that even guarantee critical hits when they're used!
Implementing critical hits is a design decision that usually takes a level of direct control away from players (and even the game's AI) and puts it in the hands of fate. The upshot of this is that combat is more exciting and challenging, with players having to plan for taking sudden, unexpected bursts of extra damage while simultaneously enjoying the same possibility of knocking off an enemy or obstacle with a sudden fortunate attack. The downside is that it requires more careful attention from players and makes fine-tuning a game more difficult for developers when balancing that game's various challenges, especially in games with an important critical hit system.
Though the primary purpose of most critical hits is to provide for more powerful attacks that are decided by randomizing in the game engine rather than the player's choices, critical hits can be decided by a number of factors, some of them within a player's control, with each game being different.
Many games that feature critical hits have a predetermined, baseline chance to score a critical hit, with 5% being a popular number. Often, but not always, this will be the same number for the player and AI opponents. This baseline chance to critical hit can stand by itself in a game, or be modified by another factor.
Improved Crit Chance
Improving Critical Hit Chance in a game directly affects a chance to critical hit, making other possible outcome less likely and replacing it with a greater critical hit chance. Often this can be controlled by the player when selecting equipment and modifications, or assigning abilities in Role-Playing Games.
Improved Crit Chance from Statistics
Games that derive a higher possibility for a critical hit from a character's statistics use an algebraic formula to determine how much extra percentage chance for a critical hit comes from each point of a character's statistics (Often Luck or Dexterity, but almost any statistic effects critical hits in one game or another.) Characters with a higher score in this statistic have a higher chance to critical hit.
Power Level Crit
Some level-based games use the baseline critical chance (above), but modify it according to the level of the combatants, adding a further sharp advantage to higher level characters and enemies. The modifier could be simply be a 2% increased chance to critical hit per level difference between combatants, so in the 5% baseline example, an enemy with one more level than the player would have a 7% chance to critical hit. This extra (or reduced) chance has nothing to do with statistics or other aspects of a game, though those other factors can affect the critical hit chance as well.
Character Choice Crit
Miscellaneous decisions, like picking a certain weapon type or character, can effect the chance of a critical hit, independent of the other factors above.
The 1975 role-playing game Empire of the Petal Throne introduced the concept of critical hits. Using these rules, a player who rolls a 20 on a 20 sided die does double the normal damage, and a 20 followed by a 19 or 20 counts as a killing blow. According to creator M.A.R. Barker, "this simulates the 'lucky hit' on a vital organ."