A couple non-obvious things about your stats that the wiki will tell you, IF you know what to look for:
- Your accuracy has some impact on your bullet spread, yes. But it is primarily your chance for a 100% perfect shot REGARDLESS OF RANGE. Yes, if you fail that roll when you're right next to the alien you are more likely to hit it anyway than if you're far away, and yes, a better accuracy means it won't stray as far so you're more likely to hit. But when you see a guy with 50% accuracy miss half his shots from five tiles away, don't be afraid to make him a sniper from forty tiles off anyway...he'll still hit half the time! You do NOT need to brawl at close range (IE alien-grenade range) to hit your targets, as long as you have a clear line of sight, and I'm pretty sure that even darkness and concealment don't matter as long as you have a spotter. If you really need to hit, kneel and take that shot. It seems like a waste of TUs, but even from across the map, you WILL hit.
- Your initiative is Reaction * TUs. Well, % of TUs remaining. "I left enough TUs for a snap shot, I should be able to take one on the aliens' turn!" Well, yes, but if you left even more TU, your chances of firing before the alien kills you go way up, because you react quicker!
- Initiative matters on both offense and defense. If you know there's an alien sitting just inside that door, the instinct is to open it with some schmuck that's almost out of TUs and then move him out of the way. Bad idea! He's got a shitty initiative (his TUs are low), and he'll get wasted. Your instincts will also tell you to leave your high-reaction guys sitting there to watch the door, while your low-reaction guys take point (since they won't act as guards). Bad idea! Your highest-initiative guy should take point whenever you know you're moving into the line of fire. If you walk through that door with a reaction of 70 and 90% of your TUs remaining, you've got a decent shot of getting a jump on that floater. Most aliens have wicked-high initiative (reaction of like 70), but if you walk through the door with high initiative yourself, they'll only get to take ONE shot, instead of like...three. Oh yeah, and those high initiative scores are on Beginner. It only gets worse from there.
- Oh yeah, if you're planning to use reactions defensively, make the alien come to you. If it's already spent 20-30% of its TUs by the time you see each other, and your guys have full TUs, you will probably get the first shot. If it only takes one step before it sees you, you're going to suck plasma.
- Difficulty. Aliens have 50% accuracy and armor on Beginner, and 100%+ starting on Experienced. Are you playing on beginner? Do you think the aliens' armor is too high, and their shots hit too often? Heh. Heh. Heheh.
Umm...For average everyday alien missions, I know they get score:
- for spending time flying over a region
- extra for spending time landed in a region
- bonus for lifting off from a landing site
So if that's what you're worried about, don't worry too much about them landing.
Here's my best understanding: When the aliens decide "Let's start an abduction mission" or whatever, they queue up some ships in a certain order with a certain vague timetable. They proceed in this order NO MATTER WHAT. Shooting down earlier ships can postpone later ones, but you can never convince them to abandon their mission.
Naturally, if you shoot down a base ship on the way to establish a base, or a terror ship on the way to drop off its cargo, you can stop them from actually performing the mission.
Pacts are much worse. To prevent a country from signing a pact, word on the street is that you can't let any scouts of any size from that mission TOUCH THE GROUND. That includes crash-landing, even if you clean up.