Do task difficulty, specially for spell casting, affect how frequently my skill increase?

Or putting it differently: For power leveling a skill, am I better off casting light level 1 or light level 7?

The controlling attributes define how much you gotta do (higher attributes the quicker.) but it's the total number of spellpoints used in casting that gets that next realm/spellbook point. So casting Light at lvl 7 is always quicker than casting Light at Lvl 1. Seven times better. Also, doesn't matter if the cast is successful or fails. You could randomly fail every spell roll and you'd get there just as fast from 0-100 training with every one as a success. So, in short, more spellpoints for the speedy powertraining!

Skiil increase does not go up 7 times faster if you select to cast spells for training at PL7 instead of PL1,
so its certainly not only based on the spellpoints you put in.
You can get statistic proof on that in reasonable time with divine trap spell.
Additionally mana is limited, so the high PLs will force you to sleep more or jump to a vendow who refills the mana stones if its about training during gameplay.

It still *does* increase faster. My theory for skill increases is as follows:

When you exercise a skill, you get an amount of skill experience based on the difficulty of the task. For spells, it is likely proportional to the number of spell points spent.

Then, after each time you get skill experience, it is compared against a random number to determine whether the skill increase actually occurs. Hence, the chance of a skill increase will increase with each use. When the skill increases, the skill experience goes back to 0.

When you use a spell at PL7 instead of PL1, you get perhaps 7 times the skill increase, but only one random roll to see if the skill actually increases. Actually, the rule for many non-combat spells is a bit different; when you cast certain spells (Light being one of them), a practice timer for the spell will be set, regardless of whether the spell succeeded or not. If you cast the spell again (even if by a different character or via an item instead of a spell), and the practice timer has not run out, it does not count as practice. Whether or not a previous cast of the spell is in effect is irrelevant here.

Note that practice timers are not saved when you save the game, so you can clear them by saving and reloading, allowing you to get more practice in. (This is a useful strategy if playing on a modern computer, but on period computers (those common around the time the game was released), the long loading times make this strategy impractical.)

Note that other spells have different rules; for example, many combat spells like Bless and Superman don't count if used in fake combat; there has to be an enemy present at the start of the round for the spell to count as practice. Stamina only counts if used during combat, but fake combat counts here (unlike with Bless). Actually, it *does* matter whether the cast is successful; a failed cast will give you *more* skill experience than a successful cast. Hence, if you randomly fail every spell roll, you will get to 100 faster than if every one were a success. (The manual actually hints at this fact.)

Note that this is unlike Dungeon Master (where failed casts give less experience) and Morrowind (where failed casts give none).