If a character is hit by a decrease attribute spell and subsequently hit with an increase attribute spell, which attribute is the target number based on?

There are two answers, dependant on whether you want PCs to do this intentionally.

Easy abuse: The TN for the increase is the number after the decrease and the full incease remains even after the decrease is dropped.

Hard to abuse: The TN is the current attribute, but also record the successes against the raw number because that's all that will count when the decrease is dropped.

From the Shadowrun FAQ:

Adding, perhaps, that there's some serious difference of opinion about the Shadowrun FAQ

I think that's what the FAQ is saying. Alternately, if a Cha 1 troll has a Increase Cha 1 spell on him, he can't get Decrease Cha. The FAQ bites both ways.

See, I read that as only the highest force spell is taking affect, effectively countering the weaker spell.

And I'd have Increase spells a target uses the base attribute of the character.

Say target X has a 4 STR and is targeted by a force 3 decrease STR spell. 4 successes leave him with 1 STR. The Target's friend with a force 4 Increase STR spell tosses it on him immediately following, against a TN of 4 (the target's normal attribute.) Thus eliminating the force 3 spell and increasing the target to STR 8.

So,
If we follow this line of thinking, a grade ten initiate could not successfully cast increase/decrease attribute on someone after a "starting" character cast the same spell? That seems, at best, unlikely.

I can see the grade ten simply dispelling the first spell before casting his own to get around this. I don't see a blanket "can't do it" as reasonable. Why, and I understand we are talking about a make-believe world where the rules are arbitrary creations of one group of people, would one spell "lock out" a particular aspect of the target?

I think you have the right of it Nikoli. It allows for more powerful mage to overpower weaker ones and also allow weaker ones to have a chance of success vs more powerful mages.

If you want to use the base Attribute as the TN, then it would be only consistent that you use base Attribute for all things that refer to Attribute (cybered Attributes get the shaft in this case).

If you want to use the FAQ, then it would be useful only if you allow the first Attribute affecting spell to count. Otherwise, we would go back to the original problem of Decrease/Increase Attribute again.

There is a completely different spell for cybered attribute increase/decrease in any case. (You can't use the standard ones.)

And decrease att. spells use 10-essence for the TN, which makes more sense as it's technically a health spell (though the same should be true of increase att. spells).
Increase uses the attribute being modified, which is silly. Why should magic give a rat's ass how stong someone is? t\The armor spell doesn't use your current Ballistic and Impact total for it's TN.

My point is that you need to be consistent.

If you are taking Attribute to mean the base (unaugmented) Attribute, then you need to apply this rule to everything refering to Attribute. This includes Unarmed Combat, Initiative, etc, because everything refering to an Attribute refers to the base, which was what I meant by cybered Attributes getting the shaft.

Increase/Decrease Cyber Attribute would then refer to the base Attribute of the cybermodified Attribute as per spell description, which opens up another can of worms by making the Incr./Decr. Cyber Attribute spell too powerful.

If you wish, you can say that ONLY in the case of Incr/Decr (cyber or not) attribute, the TN refers to the base attribute. Then yes, make it a very specific house rule. I prefer general rulings that are applicable in any situation myself.

I think the best way to handle this is to require that the first spell be removed before the second one can be added. Which brings up a question.

How does one break a spell that is currently being sustained by the mage, other than geeking the mage doing the sustaining? Is there any rules for a dispell, or interupting the mana flow or anything like that that I've missed?
If not, this seems to be a pretty big hole in the rules.

errrr, yea, sorry about that

So, my ruling, until I get an official ruling, is that you must dispell the one there before you can add another spell affecting the same stat. This avoids abuse and feels consistant.

There is balance and then there is logical balance.
Why should lowering an attribute be more difficult when most fluff concerning magic states that making bigger is easier than making smaller (see the stuff on expanding the radius of spells). Why should a spell be more difficult to cast on one willing target from another? Both are willing targets, spell should be easy to land, regardless of what it does. The most unbalancing aspect of increae attribute would be Increase BOD, as this has several direct game consequences. Guess what, there is a reason why when in doubt you shoot the mage [first AND/OR a lot], this just adds to that. Magic is the trump card, the plot hole, the ace up the sleeve. It is by it's very nature unbalanced against everything else in the game, except magic.