Here's your Eve Tip of the Day on stacking penalties. (thanks to Saki Desuyo!)
As we usually advocate, it is best to have your ship do one thing well. However, there are limitations on what you can do associated with improvements. Because the overpowered nature of some of these abilities, CCP introduced stacking penalties. They only apply to certain modules and not others, and you see it more in the effect than anything else. Note, 'CCP Math' is a thing here.
Full list of this https://wiki.eveuniversity.org/Stacking_penalties
Generally the way this works is:
- The first module that increases an individual stat does the full amount listed
- The second does a little less but near full amount
- The third module adds even less
- The fourth module adds very little
- The fifth module adds nearly nothing.
The most common one we experience early on is the Vexor. You'll note that the Drone Damage Amplifier is affected by stacking penalties. We recommend 3 DDAs. You can see this by the raw damage (max skill, 5 drone for drone lands):
- no dda and no skill - 167 dps, Vexor gives skill per level of drone cruiser
- no dda - 250 dps, 49.8% increase by Cruiser 0 to Cruiser 5!
- 1 dda - 302 dps, 20.8% increase
- 2 dda - 356 dps, 17.9%
- 3 dda - 397 dps, 11.5%
- 4 dda - 420 dps, 5.8%
- 5 dda - 429 dps, 2.1%
This is because all of these modules affect a single stat, the drone's Damage Modifier. The strongest effect will take effect first, so that's why you might have some fits with a single faction or stronger module. This also works with scripting/non-scripting modules.
To help yourself, you try to amplify other helpful stats, like instead of just raw damage, maybe tracking or speed. This is why most ships have 1-2 affecting a single stat, and instead push up non-stacking penaltied things. Like Hardeners is stacking penalty, but size of your raw shield isn't.
So, when asking yourself, should I do a max rack of something: what else could help you there? Every mid/low slot *should* be filled.