Progressing through the overarching campaign of Mass Effect 3 affects two primary statistics that impact the "end-game" players will receive: "Galactic Readiness," and "Effective Military Strength." Shortly after the game's release, some confusion arose about Mass Effect 3's multiple endings--specifically what one would need to do to achieve the game's "best" ending. Numerous claims from users arose, who believed that it wasn't possible to be completely prepared for the game's finale without boosting "Galactic Readiness," which can only be accomplished by playing the multiplayer component (or the iOS game, Infiltrator). Not true, says BioWare.
"You do NOT have to play multiplayer to get the best single-player endings," the official statement on the BioWare forums notes.
"Effective Military Strength" is the statistic that determines a player's success during Mass Effect 3's end-game. Collecting "War Assets" during the single-player campaign boosts this rating. Furthermore, there are more assets that can be collected than are necessary, but a certain threshold must be reached to get the "best" ending.
"Galactic Readiness," on the other hand, isn't even impacted by the single-player campaign at all, but increasing the stat by playing multiplayer (or Infiltrator) will reduce the number of War Assets one needs to collect in the single-player campaign to be successful.
"If you play a lot of multiplayer, you will need less War Assets from single-player to fill up your [Effective Military Strength] bar (i.e. it will balance out the requirements to account for you playing in both modes)," senior marketing manager Jarrett Lee states.
In summary, "Effective Military Strength" is the stat that determines Mass Effect 3's end-game proceedings, and it's a stat that "can be maximized via collecting War Assets alone, even if your Galactic Readiness is 50%."