Miss chance is an influential statistic, but it is neither documented nor ever
shown in the game. You will need to use Mad God's utility to see this. It is
the largest contributor to accuracy in physical combat, and is actually
somewhat more important than weapons skill. It does not affect number of
attacks or damage; those are primarily mediated by character level, Dexterity
and Speed (number of attacks), and Strength (damage).
Miss chance works like THAC0 from AD&D 1st and 2nd edition, except that the
important die roll uses a 100-sided die, not a 20-sided die. Miss chance for
every class starts at 100 for every character, regardless of race or class. It
is modified by weapon skill, the to-hit modifier of the weapon used, the
specific attack mode you employ, and the effects of status conditions like
Irritated. +1 to hit appears to be the equivalent of -5 to miss chance. Like
THAC0, the lower your miss chance is, the more likely you are to hit a monster
in combat.
Miss chance can go down if a character gains a level, but only if the following
three criteria are met: (a) The level attained is not higher than 20; once a
character reaches 21st level, that character is forever barred further miss
chance reductions. (b) The level attained is not lower than the highest level
the character has ever achieved. (c) Base miss chance may not drop below 0.
Example: You create a brand new. At level 1 she starts with a miss chance of
100 like everyone else. You advance her to level 8; at each level-up, she gets
a reduction in miss chance (1d4+1 from being a Valkyrie). You then switch her
class to Psionic. While her level drops to 1, her miss chance remains
100-7d4-7. You then advance her to level 11. Her miss chance does not change
until level 8, when she gets a second level 8 miss chance reduction (1d3 for
being a Psionic). The level-ups from 9 to 11 also each reduce her miss chance
by 1d3, making her miss chance at the end of all this equal to 100-7d4-4d3-7.
If you are minmaxing your characters by changing class multiple times, this
ability to double-dip miss chance reductions at maximum achieved level is
something to keep in mind: if you are doing many class changes you should try
to do so at exactly the highest level that character has achieved. This target
can go upward as you reach new areas that give more experience. The game
however sometimes messes up keeping track of what your highest level achieved
is, which makes getting miss chance reductions easier (the bug described in
section 1D(6)).
Miss chance reduction by class is given in section 4, but to summarize here:
Class | Miss chance reduction at level-up
----------------------------------------+----------------------------------
Mage, Alchemist, Psionic, Bishop | 1d3 (1 to 3)
Priest, Thief, Bard | 1d4 (1 to 4)
Fighter, Ranger, Valkyrie, Lord, Monk | 1d4+1 (2 to 5)
Samurai, Ninja | 1d4+2 (3 to 6)
