It's like going shopping. "EVERYTHING IN THE STORE IS 50% OFF! Sale items take an additional 25% off."
You take away 25% of the interruption possibility when you get hit. So, if you get interrupted 50% of the time, then it drops you down to getting interrupted only 37.5% of the time. Add another wand and it drops your interruption down to 28.1% chance, etc. Druids rope would be another 20% down, so that's 5.6% of the total, so you get down to 22.5%. WHMs out there with AF duckbills on will get another -20%, so that takes you down to 17.9% chance of getting interrupted.
The only way you'll be sure to not get interrupted is a piece of equipment that gives -100% interruption, or to simply use Manafont. Either way.
You got the right answer, but it could use a clearer explanation:
For the sake of argument, let's say you could equip this wand on every equipment slot you have, or (more realistically) you had multiple pieces of interruption-down gear. Now, if you simply subtracted all of the percentages from a worst-case 100% chance of interruption, you'd end up with something like -200% chance of being interrupted, or in other words, a 200% chance not to be interrupted. That makes no sense at all. You can't have less than a 0% chance of being interrupted or more than a 100% chance not to be interrupted, and the servers will probably explode just trying to calculate the proper number. Therefore, the "let's just subtract all the percentages" theory doesn't make any sense.
Let's start out with an imaginary 100% chance of being interrupted while casting spells. I should think it doesn't accurately reflect the real game, but it'll give us a nice, round number and will give us a worst-case scenario.
Now, we put on our first wand. We take 25% of 100% OFF of 100%. It may seem a bit odd now, but it'll be apparent why you do it later.
100% - (100% x 25%) = 100% - 25% = 75%.
Now, let's put on our second wand. Now, we're only left with 75% from the first wand. We can't suddenly say that we're back to 100%, because that would mean that we never put the first wand on in the first place or that the two wands are acting in separate universes. So, we have to take 25% of 75% OFF of our new 75% left over from the first wand.
75% - (75% x 25%) = 75% - 18.75% = 56.25%
Just for ***** and giggles, let's throw in that Druid Rope the original poster suggested, at a 20% decrease. Now, we already have both wands going, so we'll have to use 56.25% as our base percentage. Again, take 20% of 56.25% off of 56.25%.
56.25% - (56.25% x 20%) = 56.25% - 11.25% = 45%.
So, now, we've ended up with a 45% chance of getting interrupted while casting spells. Sure, it's not the 30% the original poster had in mind, but it's a mathematically sound number. Note that no matter how much of this kind of gear you put on, you'll never reach the magic 0% chance of spell interruptions, but you'll get closer and closer. Moreover, you won't hear any news reports of servers that spontaneously combusted. Also, consider, that it doesn't matter in what order you put on your duds; you'll still come up with the same answer.
Edit: I just remembered something... the 45% we ended up with is a worst-case scenario. This means that, if you were getting interrupted 100% of the time before, you'll only get interrupted 45% of the time now. In practice, I probably don't get interrupted 100% of the time, so the final number's bound to be lower than 45% (yet still never 0%).
Don't forget that the damage done to you with a single hit determines how likely you are to get interrupted.
Also, getting hit multiple times stacks the interruption rate.
And, getting hit during different parts of casting a spell will yield different interruption rates. If you get hit at 35% complete, you'll be much more likely to get interrupted than if you get hit at 91% complete.
This is more based on casting mechanics than interruption rates. In general a spell will only be interruptible when it is between 20 and 80% casting. There are some variation depending on if you have fast cast (which can lower it to as much as a 20-45% window), the casting length of the spell (Very long spells like Flare may have different rules), or possibly some other variables like debuffs. It is very common practice to cast a spell right before a mob will hit you so it hits during the first 20% and will not interrupt, giving maximum casting time.
So, what if you were to sub NIN? You would have the duel wield effect and utsemi being able to duel weild these for a 50% doen iturruption rate so being able to cast utsemi would be alot eisier
good for RDM/NIN soloing I'd think ^^ but, a lot of people soloing as RDM/NIN would likely prefer higher-damage weapons over these, even with the spell interruption rate down. would prolly be good for NMs or HNMs as RDM75/NIN37, but again, the damage is so low, you'd likely stand more chance of death due to the time it takes to just kill the mob vs higher damage weapons and more chance of interruption. with utsusemi and blink, a RDM shouldn't get hit anyway, and with stoneskin, there should be little chance of interruption anyway. just my opinion though :P
The one thing I don't get about this and wands in general is that they seem to be always so cheap. I picked up this little item in sandy for 2,000 gil. Far too many WHMs are underestimating the benefits of stat and defence boosts of a good wand and shield combo and going for staves instead, which only boost HP and MP generally. But since no-one wants them, the prices are pleasantly low.
This wand probably isn't in as much demand since at level 28 a Yew Wand has better stats. Spell interruption isn't an issue with in a party with a good tank and if your soloing you'll probably want a weapon with better damage.
As far as the arguement about subbing NIN with WHM... I for one sub NIN with my WHM when I farm for dual weild and Utsusemi... and how dare you, whoever you are, tell people how to play their game. If there were any jobs that absolutely should not be combined under any circumstance... SE wouldn't have allowed the free choice that they gave us. If you decided to use RDM as a sub for your MNK, why should we care... that's the way you want to play. They weren't talking about being a WHM/NIN in a party for dual weild moron... they were talking about for farming purposes. Read the post all the way through before you go ranting about what job sucks and doesn't suck.
Now that I went completly off-topic, let's get back to the matter at hand. Yes, this wand's spell interruprtion rate does stack, I have been hit while equipping one Hermit's/Eremite's Wand, and been interrupted before... where as I have dual weilded these bad boys and have yet to not get a cast out ^_^ Happy Hunting!
P.S. it doesn't give you Spell Interruption Rate -25%... it would give you a total of -56.25% Hope that answers some questions.
While this sounds very promising, there has been one thing in the back of my mind for a long time concerning anti-spell interruption that I have not had the chance to test. Another way to interrupt a spell is by moving. I feel somewhat confident that this stat is also raised by building the corresponding magic skill, and less often triggered by using lower level magic. My question is, have you noticed a significant, if any reduction in interruption from casting on the move? If it is possible to increace or guarantee any casting on the go, countless new possibilites spring to mind. ^_^
the only kind of 'Casting on the Go' I know if is Fast Cast. By the time you have Fast Cast 2 or 3, you will be able to move at the end of the cast when it's still counting up.
Just adding confirmation to what was said... you sub ninja on whm to dual weild for several reasons... for wands, it's to multiply the stat bonus, for warhammers to increase the damage. I haven't confirmed yet, but I beleive the specials would also double.. but I'm not certain it would be a direct stack for a 50% decreased interruption rate, I believe it would decrease by 25% then decrease the rate again by another 25%.
WHM/NIN? That's easily one of the worst job combinations imaginable.
Look, as a WHM, you're not supposed to be taking hits. When you are, it's up to you tank(s) to get it off of you. If you manage to squeeze out some cures on yourself while taking heat, that makes it harder for the tank to remove the mob from you. Be patient, have someone else heal you, or switch groups because this one will get you killed. Anyhow, cure between blows, it's not hard.
2 of these wands will give you FAR less than subbing a caster job, in terms of both stats and skills. On top of that, you can wield a single wand that gives you +4 MND/INT at this level, and a shield to help defend you when you do get tagged (it's not much, I know, but better than nothing).
Now, if you could manage to heal yourself a little more after using Benediction or some other ultra-high aggro action, you might buy time for your party to kill the mob before you go down. Of course, that would happen far less often if you had more MP in the first place, to heal your party without resorting to Benediction. So doing WHM/NIN in anticipation of such an event is a self-fulfilling prophecy.
If you want to worry about interruption, play NIN or PLD.
thank you finally someone else belives in free will, and not being a "cookie cutter job". I hate when people tell me that im not playin my Rdm right. I want either Rdm/Nin for Duelwield, or Rdm/Smn for, besides once i get the avatars, extra mp and Auto Refresh when i hit 50Rdm/25Smn so i can keep the spells coming longer witout endin up oom in the middle of a fight. Again, thank you