Many ship fittings in EVE Online state in their description something along the lines of:

Penalty: Using more than one type of this module or similar modules that affect the same attribute on the ship will be penalized.

What is the nature of this penalty? Is it a hard-and-fast mathematical rule? Will using, for example, two of an item be less effective than using one, or is it a case of diminishing returns?

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1 Answer

From the UniWiki on Stacking Penalty: ( A broken record, I know )

The Formula

The stacking penalty for module number n is S(n) = 0.5^(((n-1) / 2.22292081) ^2)

The Numbers

1st mod: 100.0% effectiveness

2nd mod: 86.9% effectiveness

3rd mod: 57.1% effectiveness

4th mod: 28.3% effectiveness

5th mod: 10.6% effectiveness

6th mod: 3.0% effectiveness

As is clear, stacking more than 3 or 4 modules or rigs - unless you really have nothing else at all that you could fit there - that affect the same stat is fairly pointless, as your benefit is so tiny.