how often do I miss?

and what are the contributing factors?

You miss if an enemy strafes out of range of a projectile, or if the enemy has an evasion skill active. The % chance is based on the skill, and whether the strafe is successful or not. Usually, you'll miss when a melee is heading towards your party sideways and changes direction midway.

sorry --- should have clarified.

I meant melee attacks w/o regard for defensive skills.
I thought I had read somewhere that you don't always hit, and wondered if my warrior occasionally missed w/melee attacks.

if their standing still and not using any skills to evade/dodge/block... all ur attacks should hit

You can still hit as long as you make the action even if your target is moving

melee attacks do not miss if no skill/condition/hex is active

however, you can miss attacks due to :

condition : blind
hex : for example price of failure (necro), spirit of failure (mesmer)

the target can dodge and evade attacks with :
stances : for example dodge (ranger), shield stance (warrior), distortion (mesmer), ...
wards : ward against melee (elementalist)
enchantments : for example aegis (monk), guardian (monk), ...

also, your attack will hit as long as the target was within melee range when you started the attack

You can also get body-blocked by allies/pets/npcs/etc so as to not be able to get close enough to attack. It can be hard to tell sometimes when too many allies and enemies are involved.

well, duh. but that has nothing to do with missing if it doesnt even involve initiating the attack.

~lothar