User blog comment:DynamitMC/What the Drop Rates Actually Mean/@comment-26486243-20160221092258

I'll just leave here three additional notes that Kaga and 37.5.40.152 have not mentioned yet.
 * The S-rank rate matters a lot, since the drop roll is only made on S-ranks. Even if a higher difficulty has a higher drop rate (which isn't always the case, since Akizuki drops more often on M Med than Hard), you need to multiply the drop rate with your S-rank rate first in order to get the rate you plug into the equation (1-P(A)'). For instance if the drop rate is 2% on Amagi but you only S-rank Medium 50% of the time, your effective drop rate is actually 1%. This factor is why Amagi drops less often on the whole on Medium than Easy even though they have an identical drop rate if an S-rank is obtained - it is that much less likely to get an S-rank.
 * Resource costs add up a lot (mentioned only briefly in the post). Because you'll have passive regeneration and resource expeditions as a base line, using something that has half the resource cost does not mean your resources go down at half speed - it could even remain unchanged at low enough consumption levels. While in clearing we are concerned with maximum viability/power for getting the kill, in farming we are concerned with minimum resource costs per attempt. Just because Yamato can do a map does not mean Yamato should do a map, especially for farming. In general, CAs, CLs and DDs are the core of farming compositions, and BB will appear either sparingly or not at all.
 * While the aggregate probability of getting a drop increases over time, the probability of each attempt is independent. At 180 S-ranks, you are no closer to the drop than you were at 10 S-ranks. Should there be a conflicting goal to the farm (say, clearing the event on All-Hard before it ends), you will have to set a stop-loss time limit, and once that limit is reached, give up and pursue the other priorities. While it feels like you might be 'wasting the effort' on someone you spent several hundred S-ranks on already, the reality is that the effort is wasted every time the drop table does not roll the fleet girl you want. Knowing when to stop merely prevents you from losing more. Of course, if a particular fleet girl is your highest priority above all else, then you will have to be prepared to sacrifice every other objective in her pursuit. That's what it means to be the highest priority.