I have investigated combat in the combat window, and it is obvious that attacks are 1-based rolls and defenses are 0-based. This is based on the 1.1 beta. For example, send a fleet of 2 attack/1 defense against a 3/0 fleet. You will "never" see any of the ships in the first fleet do 0 damage, but you will in the second. And there is exactly one line for every ship that fires on its turn, so it can't be that it is just not showing the zeros.
Admiral Bridgehead
I fully agree with the above comment. In fact, I ran combats and kept track of the rates of damage dealt from an attacker=3/defender=1 scenario, and the distribution was in complete agreement with a distribution one would expect if the attacker is on 1-based rolls (note: you have to be careful with the stats of lethal shots as the number shown might not be how much damage was dealt... but you can't just ignore them or you'll have a selection bias against larger damaged numbers).
As such, the combat equations need to be adjusted:
And while we're at it:
