Experience

A soldier earns experience during combat. After the mission, the soldier will then earn a random amount of stat increases based on what experience the soldier acquired during combat.

How Experience Points Are Applied

Soldier stats can be divided into two areas, primary and secondary stats.

Primary Stats

• Primary stats only increase when particular actions are performed. To see exactly how to trigger each stat, follow its link. The stats are:
  • Psionic Skill
  • Firing Accuracy
  • Reactions
  • Throwing Accuracy
  • Melee Accuracy

Note: A Primary Action is any action that increases the above stats. 

• The number of primary skill points you earn at the end of a combat mission relies solely on the number of primary actions you perform during combat. Your current skill level doesn't matter. It doesn't matter if you have the lowest recruit value, or you're one point below the cap.

• Points assigned have a stairstep function, corresponding to a range within which points are rolled. The range increases as more actions are performed:

  Primary   Roll   Average   Ave. Inc. /
  Actions*  Range  Increase  Min. Actions     *Successful Psi actions count as 3 actions - see below
    1-2      0-1     0.5      0.50
    3-5      1-3     2.0      0.67
    6-10     1-4     2.5      0.42
    11+      2-6     4.0      0.36

• Performing more than 11 actions doesn't cause any more increase. This was tested out to 200 actions. Once you reach 11, you get a 2-6 roll, and that's that.

• As you can see, there are distinct breakpoints at 3 actions and 11 actions, corresponding to an average increase of 2 and 4 skill points, respectively. Three actions are the best "bang for the buck", if you have a limited number to spread across your squad (a.k.a., a limited number of aliens to kill). It's not an issue with psi or throwing (which you can do all you want), but is for other primaries.

• Related to this, letting your soldiers throw 3 times just before ending a mission maximizes Throwing Accuracy growth vs. your personal time spent. (If you give a damn about Throwing!)

• In a sense, more than 3 actions wastes those actions, unless you can get to 11 actions. (Arguably, getting to 6 is an exception, especially if you find yourself at 5 actions after e.g. some reaction firing.) Unless you have plenty of targets (so that many soldiers can make it to 11 actions), try to stop at 3 actions. Either count bullets as a crude approximation, or use a utility that reads the UNITREF.DAT experience counters from a saved game for exact counts. The number of bullets you've shot will accurately reflect your Reaction counter (unless the person also did direct fire sans reacting), but knowing how many hits were landed for Firing Accuracy can be much more problematic. For a little more detail on efficiency versus experience, see this.

• Psi actions are a special case. Successful panic or mind control (MC) attempts cause the psi experience counter to go up by three actions. Thus, you only need to do 4 successful MCs or panics to get all the psi skill points you're going to (4x3=an experience count of 12). But unsuccessful attempts only cause the counter to go up by one. So you need 11 failed attempts to get to the 2-6 roll range, just like for all other primary stats.

• Throwing Accuracy is also an oddball. While it follows the rules for all primaries (except Bravery), it only increases your Throwing Accuracy. It does not trigger secondaries to increase. (It would've been too easy to increase stats, otherwise! Actually, Psi is just as easy to develop, but it takes time to research psi, and then it needs more combats to reach its cap.)

• Performing more than 255 actions of a given type will cause all experience to be lost. This is known to be true for Psi actions, and is undoubtably true for other actions as well (one byte fields), although Psi is the only place it's realistically likely. More precisely, experience gets wrapped around, and starts counting again. If you are seriously testing stats, or are challenging yourself by allowing only one psi person, the distance from 255 to 266 (255-11) is only 4.3%, or 3 to 4 MCs on top of 85 successful Psis (28 turns' worth, if they've been successfully MCing since Turn 1). Thus, this may sound scary, but you have nothing to fear unless you've had someone successfully MCing 3 times every turn, for 30 turns - and even then, just let them MC a few more. What's 11 more compared to 255?

• When you think about it, Ye Olde Programmers Of Yore made a simple and robust system where all squaddies are expected to participate, and hopefully each of them get to make a few primary actions that increase their score some real amount each combat mission.

Bravery

Bravery experience can also be gained on the battlefield, but by a very different means from all the other primary stats. The bravery stat itself is stored in increments of 10 (10 minimum, 100 maximum, with 110 possible for certain non-human units), and it can only increase by 10 points following each combat (it either gains 10, or nothing).

To gain bravery experience, a unit must have its Morale reduced below 50, where it stands a chance of panicking or going berserk. If the unit successfully resists panicking, it will gain a "bravery experience" point. Each bravery experience point gives the unit an additional ~9% chance of gaining +10 Bravery at the end of the combat. At 11 or more bravery experience points, the +10 Bravery gain is guaranteed.

Like other primary stats, gaining one or more bravery experience points during a combat triggers secondary stat increases.

Secondary Stats

• Secondary stats increase when one or more primary actions are performed (except for throwing). Secondary skills are:
  • Time Units
  • Energy/Stamina
  • Health
  • Strength

• Secondary skill points work quite differently from primary skills: It does not matter how many primary actions are taken; all that matters is that you did one primary action -- and your secondaries will increase all that they're gonna. It also doesn't matter if you did more than one kind of primary skill.

• The maximum possible increase falls steadily as your secondary skills approach their caps, down to an average of 1 point (range 0-2) per combat for each stat. This is actually a simplification; for a graph of skill increases vs. current levels, see here. Thus, secondary stats which start very low should increase quickly, and will then increase less rapidly as they rise. Rookies with the lowest possible starting stats will have the following as their max possible roll:

              Recruit Max  Recruit Overall
                Min   Roll   Max     Cap
  TUs            50     5     60      80
  Energy         40     6     70     100
  Strength       20     7     40      70
  Health         25     5     40      60

• The minimum possible roll is always zero for secondaries. Across the four secondaries, soldiers will show wide variations of luck or lack of luck following each combat, just like flipping coins. Still, the odds are that someone low will rise fast. It's real hard to roll a lot of zeroes in a row when the maximum is two or more (and it always is).

• If a primary skill is over its cap, it does still trigger (non-capped) secondaries to increase, by performing it.

Soldier Advancement

Given the rules of how experience works, one can predict how long it will take soldiers to advance their attributes.

Primary skill advances can be based on hypothetical soldiers who always get:

• Low experience: 3 primary-skill experience points leading to a 1-3 roll and thus, always an average of 2 points per combat
• High experience: 11 primary-skill experience points leading to a 2-6 roll and thus, always an average of 4 points per combat

Secondary skills likewise use an average roll for advancing, according to the decreasing-slope function described above.

Because you can select or reject soldiers based on their starting stats, the more important question becomes, "which stats take the longest to advance from recruit maximum to its cap?" In other words, values above recruit max can't be "bought"... so which have the farthest to develop and should be focused on, if you want to maximize your stats?

You can use this as a basis for keeping or rejecting new recruits, or otherwise choosing which skills to develop.

The table below shows the number of combats needed for a soldier to advance from recruit maximum to attribute cap, on average. A more detailed look, including starting from recruit minimum, theoretical min and max advancement times, etc., can be found here and a PDF of the same is here.

                               Experience
                                  Range
Stat           Min  Max   Cap   High Low  Secondary

Primaries
Psi Skill       16   24   100    19   38    
Firing Acc.     40   70   120    13   25  
Reactions       30   60   100    10   20   
Throwing Acc.   50   80   120    10   20   

Secondaries
TUs             50   60    80                 16
Stamina         40   70   100                 24
Health          25   40    60                 16 
Strength        20   40    70                 21

Putting It All Together

• While you might focus on the averages stated above, the actual points assigned are entirely random. The range is what you should expect for any one soldier on any one combat mission. This follows the usual X-COM behavior of defining a range and rolling randomly within it.

• Do your best to get every soldier you are interested in developing, to get at least one primary action (not including throwing). This will trigger secondary stat increases. Then try to shoot for the big breakpoints of 3 or 11 primary actions. Doing more than 11 actions does not increase a particular primary stat any more, so give other soldiers the chance to perform actions once your favorite soldiers have maxed out on theirs.

• Any soldier can (and will!) ultimately become a maxed-out superman. The worse a secondary stat is, the quicker it will rise. And you can try to target who performs which primary actions. Taking advantage of the breakpoints of 3 and 11 can also help you manage potential skill increases (a.k.a. targets) as effectively as possible. That said,

• Early in the game, don't spend much time or money fretting over stats. Any soldier is liable to come up snakes with their Psi strength. And every soldier can otherwise eventually become a superman.

• Once you have Psi on board, you can use Experience Training to optimize advancement.

• In the endgame (when you have tons of cash), if you're a micromanager role-playing to make soldiers who are maxed across the board, it's arguably more important to pick recruits who are not anomalously low in any one stat, rather than ones who are high in a few things. In other words, if your goal is 100% stats, one really bad stat may slow you down more than e.g. being a particularly good newbie shooter. Firing accuracy, reactions, and stamina have the biggest span between recruit values and work needed to max them out.

Regarding Caps

Every soldier statistic has a maximum allowed value, as shown in the table below. Your soldier will eventually have a value equal to or slightly greater than the cap. It is allowed to go over the cap, the very last time you have a combat while it is still under the cap. For example, the maximum Firing Accuracy is 120. If you go into a combat at 119, do lots of shooting, and get a +5 roll, you will walk away with 124 Firing Accuracy - but it will never increase again.

Primary skills can overflow their cap by 5, and secondary skills can overflow their cap by 1, as shown here:

Primary Stat         Min Cap     Max Cap
Psi Skill              100         105+*
Firing Accuracy        120         125
Reactions              100         105
Throwing Accuracy      120         125
Melee Accuracy         120         125
Bravery                100         100

Secondary Stat       Min Cap     Max Cap
Time Units              80          81
Stamina                100         101
Health                  60          61
Strength                70          71

*Psi Skill is the only exception to the cap rule. Combat will operate normally - you'll never see an increase from combat once you're 100+ Psi Skill - but Psi Lab training will continue to add 1-3 points each month.

You have a 33% chance of getting capped at Cap+1 for secondary skills, and you have no control over what happens (barring reloading your game and ending combat again). The chance is 33% and not 50% because there are two ways to reach Cap (being at Cap-2 and rolling 2 or being at Cap-1 and rolling 1), but only one way to reach Cap+1 (being at Cap-1 and rolling 2).

For primaries, a soldier that does 11+ actions (for a 2-6 roll) every single combat mission will get Cap+5 in that primary skill 5% of the time, on average. "Every single mission" means, without regard for the fact you're nearing the cap (and playing with the possible roll results). You can increase the odds to as high as 20%, if you use the primary increase roll ranges to inch your way up to Cap-1, then do 11+ actions on your final combat. Or you can have a 0% chance of Cap+5 if you don't pay attention and don't do 11+ actions near the cap!

Misc Notes

• With the exception of Psionic Skill training in the Psi Lab, all stats can only be trained through combat experience.

• Any combat experience earned will influence promotions. The soldier with the highest amount of experience in total will have the best chance of being promoted to the next rank. Bravery training will count towards a promotion as well.
  

See Also

• Soldiers
• Experience Training
• For "behind the scenes" on experience testing, see this