I'm trying to figure out how the hell this modifier is calculated, and what things are most important for keeping vassal "relative army power" in check. I like to play as a tall(ish) core with many big subjects, and this is one of the scaling modifiers that can be a limiting factor in how powerful of subjects you can play with. The wiki was "last verified for version 1.25" and only cites "relative army size", but experiments in the current game version are revealing that many other factors are now at play.
Here are some things I've noticed, using the console and various idea groups:
- standing army size matters, and so does force limit (quite a bit)
- maximum manpower matters, but current manpower does not
- army quality does matter, including morale, discipline, and combat ability
- army tradition and army professionalism matter -- presumably only indirectly because they give buffs to other stats
- army tradition gives a huge +25% morale, while army professionalism gives a modest +10% land fire/shock damage
- assigned generals and +general pips from offensive ideas do not matter
and now the weird shit...
- combat ability only matters if you have standing units of that type
- your unit selection matters, presumably based on unit pips
- for example, the overlord switching from condotta infantry to landsknecht infantry (both mil tech 9 units) can cause a tiny change in relative army power
- how your units are distributed matters
- having a single 15/5/0 stack is better than a 15/0/0 stack and a 0/5/0 stack
- what
- this seems to be caused by exceeding the cavalry to infantry ratio and therefore inflicting the "insufficient support" penalty on that stack (various combinations of only infantry+artillery do not cause fluctuations)
Standing army seems to be the biggest factor (unsurprisingly), followed by force limit, army quality, and max manpower. The game does seem to be somehow assessing the strength of your boots-on-the-ground stacks, taking quality buffs into account.
In my test case with a 300-dev overlord and 300-dev vassal with near-identical armies, the relative army power was typically between 64-69% ≈ 48-52% LD (LD is exactly 75% of RAP). Weirdly, the LD reduction itself seems to be the more accurate stat in terms of actual relative strength.
If anyone else wants to do some testing and maybe even figure out the formula, or if someone has already done it and I'm unwittingly wasting my time, please let me know! I only tested at tech levels 6 and 10, by applying one modifier at a time, so I'd be very curious to know if things shake out differently at higher tech levels or by stacking modifiers -- e.g. is a gigachad prussia with quality ideas and powerful mil policies, but a a modest force limit taken as seriously as they should be?
I suspect that this same calculation may also be used for other "relative army power" checks, such as when offering an alliance, so it would be a neat thing to figure out and add to the wiki.
tl;dr: standing army > force limit > army quality > max manpower
tl;dr: quantity > offensive > defensive ≈ quality > aristocratic (for reducing vassal "relative army power")