How are Combat Rolls calculated??

I know this might be a pretty n00bish question. But I realized that for somethng that is SUCH a big part of MxO - there is VERY little to nothing you can find to read anywhere about it.

I checked my manual, I checked my strategy guide, I checked the Help system in game - and nowhere can you read exactly HOW combat rolls are formulated. Does anybody know for sure? If so - can someone please post them here since I am sure the formula is different for close combat rolls, ranged attack rolls and hacker rolls.

I THINK I know how they formulate - but then what I think gets defied every day by what I see happening in my system window in game.

I'm just really curios!