How is "increases damage done by spells by up to N" determined?

is it random? or is it based on a per-spell basis?

then there is the alternative "<element> wrath" items which offer 3x more the damage (but little to none attributes). i tend to go with these. i do +250 damage with my level 54 warlock shadow bolt, as well as all other shadow spells including drain life (and the life drained).

http://forums.worldofwarcraft.co m/thread.aspx?fn=wow-warlock&t=537075&p=1&tmp=1#post537075