I'm writing to ask for help in understanding how damage is calculated. I use average damage to compare weapon effectiveness by character. I think I've made a mistake somewhere because actual damage is consistently below average.

I. Ranger. Weapon Proficiency Level is Specialized. Composite Long Bow.

Arrow does 1-6 damage, so average is 3.5.
Composite Long Bow is +2 to damage.
Specialized is +2 to damage.

Average Damage = (3.5+2+2)*2 attacks/turn = 15

II. Ranger. Weapon Proficiency Level is Specialized. Long Sword +1. Strength 18/93.

Long Sword +1 does 1-8 damage +1, so average is 4.5 + 1 = 5.5.
Strength 18/93 does +5 damage.
Specialized is +2 to damage, 3/2 attacks/turn

Average Damage = (5.5+5+2)*1.5 attacks/turn = 18.75, rounding to 19.

Thanks in advance!

Well, you seem to have the numbers right. But, going by those formulae, it seems that you're assuming that every attack hits, and that's so far from true. Especially in the early stages of the game. Your ranger may sit there for a minute or two swinging nothing but whiffs with his longsword before finally scoring.

I assume that the original poster knows how to make the game show the dice rolls. If not just go to the option menu and enable all dice rolls. You are now able to see both the attack roll (1d20) and the damage done. If you enable pause after each round, you are also able to see how many attacks a character makes each round.

If you already know this, I cannot come out with a valid explanation, except the lack of good luck.

Your damage each round (assuming you hit every time) is actually higher than your results. You have not taken critical hits (a natural 20 on the attack roll) = double damage into account.

Insane

I turned on the dice roll information recently - it's not something I bothered about before. I found it interesting (and helpful) to see what modifiers were being applied, especially for different types of enemy. Nice to see that the equipment is doing as it promises. Well worth a quick look to get an insight into what's going on - and how unlucky you are.