Quote from Moon dude
I think a blue post answered it something like this. If a mob deals 100 damage. That damage is reduced by 20% from the monk skill Resolve now dealing 80 damage. The witch doctor skill kicks in and reduces that another 20% but instead for the new value of damage 80. So it reduces it to 64, instead of 60. Resulting in a total damage reduction of 16%, instead of 20%. The more damage reduction you stack the less and less actual damage reduction you will get.
Just something i think that a lot of you are missing when you think about builds. Just like dodge doesn't stack addatively with itself neither does mob damage reduction.
I posted that already
Quote from thesikaleon
First skill its 20% of the 100%.Now mobs deal 80% of the starting damage.
Second skill its 20% of the 80%, means 16%.Now mobs deal 64% of the starting damage
Third skill is 15% of the 64%, means 9.6%. Finally mobs deal 54.4% of the starting damage.
So mobs deal 45.6% less damage