I equipped all my Spirit gear and faced a few Carrion Eaters.
-With 102 Spirit resist, I had a 72% chance of resisting paralysis.
-With 128 Spirit resist, I had a 79% chance of resisting.
So I'm pretty sure resisting follows the same logic as other resistances:
With less than 50 Spirit resistance, you resist with (score)% chance.
With 51-80, you resist with 50 + (score-50)/2 % chance.
With 81-110, you resist with 65 + (score-80)/3 % chance.
With 110-210, you resist with 75 + (score-110)/4 % chance.
Stun Immunity does nothing to your scores; for paralysis, if you fail to resist, it just halves all durations.
I let an orc trooper beat me up for a few minutes to figure out Stun, and it works similarly; it just divides your chance of failing to resist in two, which becomes partially resisting.
So, for a score of 50, it's a 50% chance of resist, 25% of failing, 25% of halving.
With 80, it's 65% chance of resisting, 17.5% chance of failing, and 17.5% chance of halving.
Once you get over 100 resistance, however, you cannot partially resist a stun. This number seems kind of arbitrary, so it's probably wrong.
Also, when halving the duration, values are rounded down--so a halved 1-second stun lasts 0 seconds.
Mod Note: avoid full quotes please!