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As Ingress Field Guide(along with many other Ingress game guides) states that each portal shield placed after the first one will have diminished effect.

Portal Shield:

A type of MOD that can be installed on a Portal to increase its defense against attacks. You can deploy up to 4 shields per portal, but each shield has diminished effect after the first one.

What I wonder is, ordering of the shields after the first one is differed or not. I mean, Placing

VR, R, C, C *


VR, C, C, R *

are differed in effect?

* VR: Very Rare, 
  R: Rare, 
  C: Common
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up vote 16 down vote accepted

A single Portal shield makes attacks 6% / 8% / 10% less effective. So if an attack without shields does 100% damage, with one VR shield the damage is 90%, and with two VR shields it'd be 81% (0.9*0.9). So four VR shields would reduce damage to 66% (0.9^4).

The effect is "diminished" because if you would simply add up the protection values, you'd end up with 60%.

Thus, since multiplication is commutative, order doesn't matter.

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