Psionic Skill is a rating of the effectiveness with which the soldier can use his inherent psionic ability.
New recruits will always begin with a value of 0 for Psionic Skill. While it remains at zero, the player will not be able to view a soldier's Psionic Strength or Psionic Skill on the Soldier screen or in battle. After a month of training in a Psionic Laboratory, soldiers' Psionic Skill rises to a value between 16 and 24 (inclusive) and their psionic stats become visible.
Psi Skill is increased by use of Psi on the battlefield, and through Psi Lab training. Both successful and unsuccessful psi attempts during combat result in improvement. Unsuccessful psi results in one point towards an improvement of Psi Skill, and successful psi results in three points, following the standard Experience equation for improvement of Primary Stats.
Psi Skill will increase from 0 to 6 points per combat mission, depending on how many times you attempt a Psi action (Panic or Mind Control, MC). With one successful psi(or 3 failures), an average of 2 points (range 1-3) are awarded. With 4 successes(or 11 failures), an average of 4 points (range 2-6) are awarded. No more skill points will be earned, no matter how many more attempts are performed.
The above numbers are for successful Panics or Mind Controls. Failed Psi attempts cause the Psi experience byte to increase by a third as much as successful ones. So Psi failures do help you gain skill points, but at a lower rate. 11 failures (vs. 4 successes) will get you all the potential skill points you can get at the end of the combat mission. For more information, see Experience.
Unlike any other skills, Psi Skill is not truly capped. Although combat increases stop once you reach 100+, the Psi Lab increases skill as follows:
End of Month Psi Lab Increase Psi Skill Range Average 0-16 16-24 20.0 17-50 5-12 8.5 51+ 1-3 2.0
It has three distinct stairsteps, and rolls randomly within each range. Rookies always start with 0, and thus get 16-24 skill points in the first month's training. If you got unlucky and only got 16, the next month's training will give you 16-24 again - if you don't earn a single psi skill point on the battlefield! That's about five combats' worth of the maximum possible average of 4 points per combat, versus the two combats' worth that the psi lab will give you for psi skill in the 17-50 range... so you might want to not exercise a 16's psi skills at all, if he won't be going on 3+ combat missions in his or her second month (but they can still go on missions and do other actions).
Also notice that Psi Lab training keeps awarding 1-3 points per month way above psi skill 100 - about 24 points/year. It breaks down at 255 because the counter byte wraps around to 0. Of course, it would take over 6 game years to go from 100 to 255, and training can always be turned off then. (Also note: stats at 170+ wrap around on the stat display screen.)
Note: Psi Lab training is lost (for a given month) if the soldier is transferred to another base, even if the destination base has Psi Lab space available. It's not the end of the world, though; one month of Psi Lab training only equals about two combats' worth of psi increase up to Psi Skill 50, and only half a battle's worth, above Skill 50 (assuming you're getting all the psi Experience you can each mission). Transferring right before the end of the month means soldiers will lose two months' worth of Psi Lab training.
Also note: The Mind Probe does not cause Psi Skill improvement. It is not an offensive weapon.
An X-COM soldier's Psi Skill has a combat increase cap at 100, but can continue to grow past that indefinitely through Psi Lab training, as stated above.
As with other primary stats, you can get a +6 roll when at cap-1 (99) and thus receive a Psi Skill of up to 105 (cap+5) from combat. For more info, see Regarding Caps. However, this hardly matters for Psi Skill, because it will continue to improve regardless via the Lab. All other skills stop at their cap.
Psi Improvement Strategies
Several points from above influence your improvement strategy. To recap:
- Psi attempts result in an increase of ~4 skill points per combat, with sufficient attempts
- The Psi Lab increases skill by ~8 points in Skill range 17-50, and ~2 points above 50
- A Psi Skill of 16 is a special case; the Lab awards ~20 skill points in this case
Some strategic observations can be based on these points:
- Early in the game, if there are few Psi Labs available but many to be trained, it is wiser to assign entirely untrained soldiers to your Labs rather than increase skills of existing psi soldiers. It only takes a couple of combats to get the increase a Lab awards, but no amount of combat will uncover additional high Psi-Strength soldiers. Even soldiers with poor combat attributes are candidates. When push comes to shove, one good psi trooper can be worth many non-psi troopers.
- Troopers with Psi Skill greater than 50 do not benefit much from the Lab. So use Labs for new Psi candidates or low-Skill soldiers until you have plenty of capacity. Also keep in mind that it will only take ~7 combats for a psi newbie to get to Skill 50 - another reason to only train new candidates, early in the game when pressed for Psi-Lab space.
- Likewise, don't worry too much about re-assigning Psi soldiers. The Psi Lab training lost is not substantial relative to the more-rapid response (or base defense) capability you'll get from sending them elsewhere. This is especially true for anyone with a skill over 50; the Lab barely nudges them.
- Soldiers with an initial Psi Skill of 16 are a special case. They receive such a boost in the next month's Psi Lab training that you should pay special attention to any new trainee that has high Pi Strength and comes up Skill 16. They can still go on missions and train other attributes - just don't give them a psi-amp unless you're pretty sure they'll get to go on 5 or more missions that month.
- At high game difficulty levels, try to have a crack team remain in Psi training even above Skill 100. By then you should have plenty of Psi Lab capacity - and may be meeting some Ethereals of awesome Psi capability. (This note pending results of Psi equation testing.)
- As a mental benchmark: If soldiers of Skill 51+ go on e.g. 5 missions a month, the Psi Lab only adds ~10% to their skill (20 pts. vs. 2 pts.). If you go on 10 missions, it only adds ~5%.
Many aliens possess Psi Strength but few use psionics. It looks like Psi Skill also acts as a "switch": aliens only attempt psionics if their Psi Skill is non-zero. Needs testing: if you equip the whole team (everyone psi-unlocked) exclusively with Psi-Amps, do aliens attempt the Exponential Mind Control exploit?
- Psionic Laboratory
- For tangential talk about Psi Lab testing, see this
|X-Com: Enemy Unknown/UFO Defense|
|Soldiers:||Time Units • Energy (Stamina) • Health • Bravery • Reactions • Firing Accuracy • Throwing Accuracy|
|Strength • Psionic Strength • Psionic Skill • Melee Accuracy • Stun • Morale • Rank • Fatal Wounds|